Oh no! I saved the Deoxys Unban Post reactions for posterity!
Few comments on it.
In the room tours I’ve used Deoxys in, it either didn’t come out or only cleaned up late game. Even with Deoxys unbanned no one even gave it a try because the metagame is so hostile to psychic types in general and people just don’t bother even using the type. Deoxys would give people a reason to try out psychic and wouldn’t be overbearing in my opinion as much as people may think. Deoxys has many clear flaws and has a really hard time even hitting the field in the first place.
Edit: If Deoxys Attack is deemed too much I think Deoxys’s Normal form would be more tame and equally deserving of a suspect.
By Wincon's argument you could say some dumb bs like unban shed tail, since the winner of the 8 person tour with super rushed teams didn't have a shed tail team. Unless I'm mistaken and he was actually spamming multiple unban Deo A tours throughout the day, the tour which had it unbanned also had like 5 other unbans including Gambit, Ursa BM, and Shed Tail, so it's not even a representation of what the mono meta would be with Deo added in.
So just to further clarify for people. When he says "no one even gave it a try", he's not even talking about a tour where only Deoxys-A was unbanned. It was an 8 person roomtour, where the people who did manage to make a team ended up with super rushed builds, and on top of it there was also Shed Tail, Gambit, and Ursa BM unbanned, meaning naturally much of what was used was shit like Shed Tail grass and Ursa BM ground. It would be a disingenuous argument even if it was just Deoxys partially since alot of people tend to not finish making teams for these tours/missing that it's unbanned, but also because any tour where only Deoxys is unbanned is naturally going to use shit that doesn't lose to Deoxys if there's genuine prep for it.
As you can see by the above lists, Deoxys has properties that would make it an excellent choice on Psychic teams in the current metagame, while also having clear irrefutable weaknesses that can be taken advantage of. While Deoxys is very strong, every single good team in the metagame runs a scarfer that can pressure Deoxys out. Not only that, almost all priority in the game can find OHKO’s on Deoxys. Even if you pair Deoxys with psychic terrain, Indeedee is very easy to pressure and take advantage of and almost every team has knock off or strong dark/ghost stabs that pressure you (Indeedee is ohko’d by knock off from Iron Treads for example). Not to mention there’s also ice spinner to just cancel out your terrain.
With the unban of Deoxys, psychic teams could really shoot up in usage and probably land around the same viability as ghost, normal or poison in the current metagame.
Gonna also have to add, no, not every good team has a scarfer. Even just ignoring shit like Poison, Stall Water, and Stall Fly, it is common knowledge to any builder that you can drop running a scarf if you believe your ability to maintain speed control with priority options and high speed tiers is adept enough. You see this with Sun Fire, for example, where the common build has Ceruledge as a stopgate as well as priority from both Cinderace and Ceruledge. You also see teams like Fighting drop their scarfer too at times, with scarfless Scrafty Fighting with unburden Sneasler being seen in last MWP, I can find the team/replay if you want though this isn't unique to just those 2 types, or even just this generation either. A personal favorite example of this is with Gen 6 Bug where even scarfless + webless offense had alot of merit, and though the meta has shifted since I considered that the best way to use bug, it's still not like you need a scarf on webs builds.
There's also a huge contradiction in your argument. You make the assumption that Psy Terrain is necessary for Deo to be broken, which is implied by the consistent focus on identifying how Psy Terrain can be handled, but the idea that Deo-A needs Psy Terrain is just not the case. Even outside of Psy Terrain it fully is one shotting every single neutral as even 100/100 mons like Manaphy are OHKOd by Psycho Boost. That being said, you simultaneously argue that A: Priority is a good answer, and B: Deoxys is psy terrain dependent or needs the terrain boost to be busted. Aside from the fact that Deo doesn't need Psy terrain as I said, there's the built in contradiction in your argument where you assume that Psy Terrain isn't active when answering Deo, considering the fact that Priority doesn't work when terrain is active.
I will also point out, you're not only ignoring the teams cooked completely by Deoxys and the lack of variety that would result from this, but are also pretty unimaginative imo as far as what Deoxys can actually run. As someone who's fought in the trenches of ladder, I know for a fact that people rocking this shit on ladder are going to be silly as fuck. When you have a mon with 180/180 offenses and 150 speed they're going to find justification in anything. If you wanted to, you could slap a band on Deo-a for example, make it 252+ attack, and have a literal OHKO roll on greninja from max hp with E-Speed. Add in some hazards and you literally OHKO or have an OHKO roll on every single offensive dark that isn't Bisharp or holding an HDB. Similarly, Rain will get absolutely cooked if there's no basculegion, even without hazards barra just gets one shot by E Speed like this. Also, even if Deo A can't switch into anything, it sure as shit forces shit out, so chances are when you have Deo vs. Water and it comes in on a specs greninja after a team mate dies you'll fully be able to get a sub off which could be GG right there depending on movepool. Deoxys A also is more powerful than Specs Deoxys-S even when Deo A has no held item, which means yeah, if you wanted to you could just scarf this shit and have a mon that's faster and stronger than Deo S while reaching many more important damage rolls. What's crazy is that even with no item you can still just one shot Flutter Mane from full even outside of terrain.
This means something you might have to account for is Scarf Deo A in Psy Terrain so uh...
252 SpA Deoxys-Attack Expanding Force (120 BP) vs. 0 HP / 4 SpD Flutter Mane in Psychic Terrain: 250-295 (99.6 - 117.5%) -- 93.8% chance to OHKO
nice scarf buddy.
Also, Deo-A can just do the funny terrain into ebutton bro shit that people spammed on Deo-S when that first released. Here's a funny calc:
252 SpA Choice Specs Deoxys-Attack Expanding Force (120 BP) vs. 252 HP / 244+ SpD Corviknight in Psychic Terrain: 196-231 (49 - 57.7%) -- 57.4% chance to 2HKO after Leftovers recovery
Literally zero switchins on fly unless you're adding a dark bird.
The next Pokemon that I think deserves a suspect

and by proxy its

form as well. Both these Pokemon would be huge boons for Psychic and would validate Psychic’s place in the metagame.
Also clarifying for the people that don't know better, no, this is not how tiering works. You don't unban the most obviously brokens of all time to try and give a bad type a pick me up.
The other question is most likely what nomadderwhat and Wincon are posing to the community: what unbans could we have that still wouldn’t change how bad the bad types are?
This is literally what we do at the start of every dlc. How do you think I ended up spamming GodGaleo with ID + CM on ladder to see what just lost on preview LOL.