Very late post time!!!
Anyways I don't really know what I want to vote since both sides present good arguments. I wrote a lot here so apologies if it seems like rambling but this is just how I see the Latios counterplay in the tier.
Latios's Counterplay on Paper
In a vacuum Latios is broken since its set versatility can be a real issue to deal with since counterplay can only apply to certain sets and if it's running the right set then it can easily punish a would be check for just daring to switch in (Minor nitpick, we should drop mentions about Choice Scarf because it's not broken in the slightest and the power drop and move locking is very noticeable).
Celesteela is probably one of the hardest and splashable Latios checks in the tier and if it didn't exist then I'm sure DNB voters would be less keen on calling just "very strong" and instead call it unbalanced. It can effectively handle most Latios sets with it's stellar typing and behemoth special defense making Latios's powerful STABs feel a lot less spammable so it forces it to rely on Mystical Fire or Thunderbolt in order to properly beat it and the move are detrimental into threats like Tyranitar and Ting-Lu so it's not always used. HOWEVER, sets such as Life Orb and Choice Specs are far more free to drop Flip Turn for Celesteela coverage which creates a serious issue for the Celesteela player since you can't predict the set on team preview, meaning that you're forced to switch into a potential nightmare especially if you aren't stacking checks. And when it's finally revealed, its coverage moves tend to sting harshly with Mystical Fire chunking you of 30% of your HP at a minimum and at the very worst you are 2HKO'd by Specs Mystical Fire after Rocks / random chip (because keep in mind Celesteela has to deal with other threats throught the game like Enamorus and Excadrill)
Ting-Lu is another strong check in theory but the reality is that it's barely a good enough check in practice unless you're running a specific Ting-Lu variant to deal with Latios. Most of the Latios sets in this tier run recovery which just makes Ting-Lu a prime target to be abused by Aura Sphere sets or at the very least slowly chipped away by Draco Meteor while it can't really do much back except click Whirlwind. Now the Ting-Lu variant in question that can actually pressure Latios is Throat Chop instead of Ruination / Whirlwind. Giving up either of those two moves is pretty bad since dropping Ruination means that you have no way to punish Enamorus, Celesteela, Skarmory, Mega Sableye, Ting-Lu check #5, etc for switching in and makes Ting-Lu feel significantly more passive than it should be in order to deal with a singular threat. Dropping Whirlwind is a little better but still mediocre since you can't even dream of checking threats like Galarian Moltres, Hydreigon, or even threats like Reuniclus and Clefable while doing worse into foes like Meteor Beam Celesteela. Dropping Whirlwind also means you're giving up a major progress forcer for Ting-Lu since it enjoys using that move to hazard shuffle. And again this is all so you can slot a medicore coverage option to beat a single mon. A mon that can still more or less invalidate you with Flip Turn sets and a common enough partner like Iron Hands or Galarian Zapdos
Now the core of Celesteela + Ting-Lu can actually be a great response to Latios and makes it feel like a walk in the park to deal with. It also helps a lot that this core is really popular since it blanket checks large parts of the tier such as Excadrill, some Mega Tyranitar variants, CM Clefable and Mega Latias, Enamorus, Iron Moth, etc. But the core itself is quite abusable by mons like Iron Hands and Thundurus-T, making building either very centralized (You can often seen Mega Venusaur and Clefable on these types of cores as a third) or forgoing the core in order to secure a better matchup into other threats.
If you're not running into Mystical Fire Latios, then Aegislash is a really good check to it and even helps to scout the Latios sets with King Shield. It's pretty hard to wear it down with Flip Turn pivoting too and it's great at punishing common Latios partners like Iron Hands and Ting-Lu as well. It fit's on every playstyle besides Stall I guess and even more offensive variants of Aegislash can still be threatening to it with Shadow Sneak. Of course switching into Mystical Fire is a pain in the ass so those sets are bound to be very frustrating to play against if Aegislash is your only check.
Mega Tyranitar is a strong answer since it traps Latios with Pursuit and only loses to the Aura Sphere variants, which aren't as common anymore. But Flip Turn Latios can still wear this down a lot so a lot of wonky positioning is required to actually trap Latios (ergo baiting it into clicking one of its STABs which it almost never will if there is a Mega Tyranitar on the field so).
Special defensive Clefable is an decent enough check to Latios since it can switch into Luster Purge fairly well and punish it with Thunder Wave / Moonblast. But the set itself is... not that great? At least in my experience since Clefable also wants to check threats like Galarian Zapdos and Mega Tyranitar and it can't do that with the special defensive sets and the set overall doesn't actually check a lot over the regular physically defensive sets since most of the special attackers in the tier already have a way to beat down Clefable (Sludge Wave / Boosted Tbolt from Thundurus-T, Iron Moth and Mega Venusaur just existing, Enamorus wiping it off the map with Choice Specs, etc). Hell even Latios, a mon it should be able to check in theory, can overwhelm it with Luster Purge stat drops.
Bulky Scizor is pretty solid as a check but it runs into a similar problem with Celesteela and Aegislash where it really hates taking Mystical Fire. Scizor stands out since it's better for more offensive teams since you're not sinking your momentum as much when you hard switch into Latios and you can regain it easily with U-Turn or Swords Dance. Latios teammates such as Iron Hands and Iron Moth abuse the hell out of this so Flip Turn variants can screw with you a lot which is not something a Scizor player wants to hear since Flip Turn is very common on Latios.
Slowking is another very solid check since you only lose to the rare Thunderbolt and Shadow Ball variants of Latios which makes it a very powerful way to scout Latios's set. After cleanly eating something like Latios's Draco Meteor, you can just Teleport into a more offensive check like Enamorus or click Future Sight or Thunder Wave / Scald to force progress. I also have been seeing Toxic variants of this mon pop up since you also punish threats like Thundurus-T and Galarian Moltres for switching in. All in all, it's a good check but Flip Turn variants annoy this as they can partner with threats such as Mega Tyranitar and wipe Slowking off the map (we call that a skilled outplay though and I'm being dead serious) so you have to more cautious than usual when playing that scenerio but it's in no way an impossible task.
Yo this is the check ever!!! Just pray that you're not facing Specs Mystical Fire or Mega Latios!!! Assault Vest Tangrowth sets seem ok on paper as a way to scout Latios and partially check it but you can only really click Knock Off in order to force progress and if it's a Mega or Z-move variant than you're doing a whole lot of nothing back to it bar maybe getting lucky with Sludge Bomb fishing. And then, as was just said, running into Choice Specs sets (or just Mystical Fire in general) is a nightmare because it will 2HKO you making it a rather poor check so you can't use Tangrowth to check Latios solely. Definitely would stack a check like Celesteela or Ting-Lu then it's fine. And again that's in no way a hard thing to do since they are great in this tier and insanely splashable.
Chansey is a really good counter to Latios but realistically it's only going to be seen on stall teams so it's not very relevant to the discussion. Although it's strong it's just way too passive and hazards prone for a standard team to want in a Latios check, espeically when Latios can just Flip Turn into a hard-counter like Galarian Zapdos or Morning Sun Iron Moth. Specs variants will also render this mon crippled for the rest of the game which is awful for a blanket special wall and more often than not stall's ONLY good special wall on a team.
Minor quip about Reuniclus because it was mentioned as a niche check, Assault Vest isn't good ever since Toxapex left and from experience it was barely a check to begin with (Stat drop from Luster + Draco = Game Over)
@ anything B- or below on the VR, you aren't mentioned for a reason: You all suck except for like really specific situations where I really need you. Not gonna mention how good of a check Mega Aggron is when it is seldom used and rarely worthwhile.
How Latios Is Dealt With In Practice
For the most part I think just having a least one of Ting-Lu, Celesteela, or Aegislash on your team should be enough defensive counterplay since they can scout and deal with most of Latios's sets on their own but you still probably need to run offensive counterplay alongside it to have outs in case you run into the wrong set or alternatively stack defensive checks (re: Ting-Lu + Celesteela). Slotting this counterplay isn't that difficult and a lot of playstyles and teams can successfully run the Defensive + Offensive Latios check core since they are pretty much used regardless of Latios being in the metagame so they fit naturally on teams.
Purely offensive checks
Greninja is probably one of the best offensive Latios checks out there and can easily revenge kill it, while also abusing Latios's partners with its own colorful movepool (it's probably also problematic in the long run but I digress). But obviously the biggest issue is that it can't switch into Latios very well unless you correctly switch into Luster Purge which is a rather risky call, so in that case you 100% need a defensive check to run with Greninja or at the very least a Draco Meteor switch in like Clefable or Enamorus. Otherwise you're always kinda just forced to sack something in order to get Greninja in safely.
Unlike Greninja, Bisharp is a much stronger check to Latios since its typing lets it switch into both of its STABs but obviously coverage moves like Aura Sphere will fuck with it so still switching in is annoying as hell. Fortunately that isn't a huge hurdle to climb especially you are running partners that pressure Latios into wanting to click its STABs intead of coverage (such as Enamorus or Iron Hands), so getting in isn't that hard and when you do you can easily pressure it out with the threat of Sucker Punch or just remove it entirely with Pursuit (massive win). Another option is just to use it as setup fodder if you're the SD variant (also great!!!).
Excadrill is pretty ass as a check and the only reason it's being mentioned here is because it switches into both STABs well and does massive damage back to it with Z-Iron Head on the Sand Rush sets. Sand as a playstyle is pretty well suited with dealing with Latios anyways with mons like Dracozolt, Celesteela, Slowking, Mega Tyranitar, etc being really common so if you're truly struggling with Latios when playing sand then you have to be throwing or something #ngl.
Enamorus is like a fake check? It hates switching into Luster Purge and is slower so most Latios variants can deal with it unless it's Choice Scarf. Thankfully Scarf Enamorus is one of the most common sets in the tier so that's not a huge issue. Although its longevity is pretty piss poor as it gets worn down really fast by rocks. Honestly that's not horrible since good positioning, such as pivoting off a Slowking or Scizor, can grant you success.
Choice Scarf users like Galarian Zapdos and Hydreigon can work well enough as a Latios check but, like Greninja, switching into it is a chore so you need to have a defensive switch in to it. Which again, isn't that big a hassle because it's not hard to fit Latios counterplay on teams.
Conclusion
I think because of the fact that a lot of Latios's defensive checks are "fake" depending on the set, the high variance of said sets making it a headache to respond to sometimes, and the semi-shakiness of its offensive counterplay,
I would vote ban. But that said, I'm not really going to care all that much if it stays legal either since this metagame has a ton of ways to counteract Latios aside from just running direct counters and the constrain it has on the builder isn't
that bad since you aren't really forced to run these to check Latios solely, but rather because they are good mons that you can just naturally fit in teambuilding. Like to the point where, I would see teams unintentionally stack checks to Latios and it doing nothing great aside from being a Flip Turn bot.