lurks in the shadows
The pressure of two layers of hazards, which are almost guaranteed against most defensive and many balanced playstyles, is often insurmountable for such teams, as a single double switch as you bring in your check / counter to a threat means that this check or counter is practically unable to check the threat you want anymore (25% damage from hazards * 2, minus the Lefties recovery from two turns, which results in a 37.5% net health loss). Are all teams supposed to use Excadrill, Latios, and Latias? Because any other slow or moderately slow Defog user can and usually is overwhelmed by the sheer offensive pressure of the 5 teammates of Deo-D + damage from dual hazards, considering that almost all of those 5 Pokemon will be usually faster than the Defog users. And this is without even getting to how many excellent offensive Pokemon exist that can OHKO the majority of Defog users and Excadrill, (Greninja, Knock off Thundurus, Knock Off + Heat Wave Tornadus-T, Head Smash Aegislash), Taunt them (Tornadus-T, Thundurus, Terrakion, Talonflame, Mega Gyarados, Gengar), or just take advantage of them (Defiant Bisharp and Defiant Thundurus). So between all those ways to make sure that entry hazards leave their mark on each game, plus the tremendous pressure offensive teams can apply with very safe double switches, any team not as fast paced is going to struggle a lot to overcome the advantage that Deo-D gives to an offensive team.Deoxys-D - I use this a lot, I wouldn't say it gives an unfair advantage in battle. First of all it has one role, suicide lead. Yes you can set up 1/2 layers of hazards most of the time but you go a pokemon down. This means your team really has to take advantage of the hazards and keep them there for this to be a sensible strategy. Starting the match 5-6 gives you fewer options and seems to be a fair trade off for getting hazards up.
Secondly defog / rapid spin can be fitted onto basically any team. Yes there are defiant users / ghost types but every team is prepared to check +2 bisharp, not to mention even with the defiant boost you have lost your hazards to the defog. Excadrill can beat Aegislash with Iron Head on the switch in so even picking air balloon leaves it to a 50 50. Overall trying to stop the hazards being removed takes work and teamslots to attempt and does not necessarily happen. You can say something like "use offensive pressure" but this is more down to the actual battle and how the opponents defog user matches up against your team.
Finally deo-d has a fair few viable counter leads, and because it will be leading nearly every time stuff like sash sd chomp, cm landorus, sub gengar, np thundurus etc can setup and proceed to take out deo-d and an extra pokemon. These are pokemon that work outside of beating deo-sharp - its not a disadvantage to be running these if you aren't facing deo d. Aegislash is everywhere and limits deo to one layer if not running red card.
Deoxys-s - I rarely use this as I like offensive pokemon on my teams to contribute to synergy/ be able to switch into at least some pokemon. Lead sets I prefer Deo-d. From my experiences facing it I would not say it is broken either. Offensive sets are not hard to handle and it can really only come in for revenge kills/ slow u-turn or volt switch. Its main moves drop its stats so even once it comes in it has to switch out again pretty soon which leaves set up opportunities/ breathing space for more defensive teams. Basically hazards / life orb mean that any of your priority users will be able to take it out while ignoring its speed, and it is not powerful to the point that offensive teams lack pokemon that can take any one hit and ko back.
I'd actually say that offensive deo-s is very healthy for the meta, without it you lose a pokemon that checks the dd boosting meta and a lot of offensive pokemon in one slot. In such an offensive meta its good to have an extra pokemon that can deal with a lot of the threats that can otherwise sweep you. Without Deo-s every offensive team will be limited to t wave thundy or a scarf ground type to be able to check dd tar, gyara and zard x in one slot. I feel deo-s balances the meta more than a lot of people realise and banning it will likely make future suspects more dangerous.
Deo-s support sets are fine for this meta as well, they really only add to the perceived danger of deo-s by giving it unpredictability, dual screens/ hazard/ rain lead are not that special by themselves. They are reliable in what they do but as with Deo-d they still leave you 5-6.
Yeah so I would not ban either.
Furthermore, the Excadrill vs Deo-D + Air Balloon situation is not quite as easy for the Excadrill player as you are describing it. Excadrill has to predict twice correctly to KO Aegislash (use Iron Head as Aegi comes in, and then use EQ if Aegi stays in), and even then, entry hazards are still up and Excadrill will need to find another turn to get rid of them. After predicting correctly TWO times. If Excadrill uses Iron Head and Deo-D stays in, Deo-D will get up a second layer of hazards. If Excadrill uses Earthquake against Aegislash after breaking its Balloon and a Pokemon that can tank EQ and threaten Excadrill switches in, the Exca players is in big trouble (Deo-D is still alive to set up more hazards, and the Deo-D players has SR up + momentum). Even if you predict correctly and use Rapid Spin as Aegi swithcs out, the opponent still has the momentum and Deo-D alive to set up the hazards again. And the burden of predicting correctly is on the Excadrill player, as Deo-D can afford to make many mistakes (set up hazards multiple times as Exca uses Rapid Spin and then inevitably you will find an opening to bring in a Pokemon that threatens Excadrill as Excadrill uses Rapid Spin, which is practically a free switch, while Deo-D can still try to set up some hazards later). And all this is if you have Excadrill, which is supposed to be one of the best ways to deal with Deo-D. Imagine how much teams without Excadrill, Latios, or Latias are going to struggle. Yeah, when using teams without Excadrill, Latios, or Latias, you can make up for the early-game advantage that Deo-D gives to the opponent with smart playing and good predicts, and come out on top, but prediction goes both ways.
Lastly, all the Pokemon you mentioned to counter-lead Deo-D are Pokemon used on offensive teams and some bulky offense teams, but most of them absolutely don't fit on any defensive-minded playstyle, from balance to full stall. Basically, only offense will be able to regularly fit in an anti-Deo-D lead without going out of their way to deal with it, either in teambuilding or in the actual battle. And this is partially why we have such an offensive metagame, because to deal with Deo-D you have to go on the offensive, otherwise it's going to be an uphill battle or you will have to overprepare for it while teambuilding.
As for Deo-S, when you downplay its two biggest advantages, of course you are not going to find it broken. Its ability to flawlessly support its team in a multitude of ways while still being able to do other roles (revenge kill and provide multiple kinds of support at the same time, which means versatility) is what makes it broken. And the ''it checks offensive Pokemon'' argument really doesn't work at all as Deo-S only really fits on offense, as it provides zero defensive synergy on balanced and stall teams. So, Deo-S is in reality only helping offense and not limiting or keeping it in check at all.
I know it's not easy to convince you and practice > words (you formed your opinion mainly through practice, and i am trying to convince you with words), but you just can't deny how offensive the metagame is and you can't deny that Deo-S and Deo-D are the biggest reasons for this, as they are almost always the Pokemon that offensive teams rely to for support. And if offensive teams can be so successful in a metagame where everyone is supposed to be overprepared for them, with very formulaic teambuilds and a lot of 50-50 decisions that basically judge the effect of the whole game (will you manage to outplay the opponent and spin with Excadrill by winning the two coin-flips that i mentioned earlier?), then there is definitely something wrong with them. Some offensive Pokemon could take some of the blame for this situation, but Deo-D and Deo-D usually contribute much more than every single sweeper can, at least from my point of view. Deo'S's screens are what allow your first set up sweeper to weaken that crucial Pokemon on the opponent's and get the momentum rolling, or even KO it and keep on rolling. Deo-D's entry hazards are what allow your sweepers to wear down their checks and counters so easily. And the same will be true for Deo-S if only Deo-D gets banned, as Deo-S will just take its place. Deo-S's revenge killing ability is what keeps your offensive team from getting swept, while simultaneously being able to switch moves (unlike other Pokemon at similar Speed levels and power) and set up hazards.
The only way i can get why you don't think Deo-D and Deo-S are broken is if you really like a very offensive-based metagame, but even then, those two do nothing but create a stale metagame, as they basically eliminate any and all competition when it comes to offensive support.