You've been very emphatic about Deo-S still in February and this confuses me as well
While I don't want to speak for him, I have a D-Speed post coming later this week. I'll just summarize a little of it for now. You can run only a + speed nature and outspeed the entire unboosted tier with no speed EV investment. This leads to the potential for a 252 attack and 252 special attack mixed set that has great coverage and outspeeds the entire unboosted tier.
Somebody, I forget who it was, mentioned that D-Speed was only good against offense. But I found in testing that the mixed set can completely destroy stall. The best coverage is Psycho Boost, Ice beam, Knock Off, and Superpower on a Life Orb. Tera Dark let's you spam more Knock Offs and live a Gambit Sucker Punch in most cases.
The only things I could find that can safely switch into it are fairy type special walls, which are basically just Clefable and Primarina, and possibly a Steel type that doesn't lose to Knock Off or Superpower. And the Clefable has to be specially defensive. The defensive set still takes 66.2-78.% from Psycho Boost, which can be a problem if it is chipped at all. Then there is Corviknight. Corv takes about 30% or so from Ice Beam, Psycho Boost, and Superpower. But it has the recovery to maybe outlast that, particularly with the stat drops and if you are being chipped with Life Orb and maybe even a Rocky Helmet hit or two.
Anyway, those are the main 3 that work against this set. Even then, there is counterplay coverage you could run like Thunderbolt, Poison Jab, Psyshock, or just straight Tera Blast if your counters became too common. But you do sacrifice important coverage to do so.
Then there is Arch. I didn't really focus much on Arch because I'm assuming it will be banned soon. But it should be noted that it only works under certain conditions. Arch will lose about 40% to Superpower, but maybe less if it has Stamina boosts up. An Arch that is uninvested in special defense will lose roughly 40%-45% from Ice Beam and 45-53% from Psychoboost, but less if it is wearing Assault Vest and/or invested in Special Defense. Closer to 30% from the special moves with Assault Vest and no further investment in special bulk. And it can also get Knocked Off.
So what this means is that, if Arch switches into a move from this set, there is a very real chance it could lose 80%-90% of its HP before it can move. And that doesn't count hazards or preexisting chip. But it depends on the set. An established Arch or one with a clean switch is more likely to be ok.
About other walls... None of them really work.
Incineroar and Ting Lu can also switch in, but neither one really appreciates the roughly 50% chip from Superpower. Ting Lu also loses another quarter or so to Ice Beam, so it can't be chipped much at all to switch into the set. I don't really consider Ting Lu to be an appropriate switch in to this set for that reason. Incin is a very niche option that requires not eating a superpower very often.
About Glowking. A standard pivot Glowking takes between 40%-50% for both Knock Off or Psycho Boost. And if Glowking is invested in more defense or special defense, the other would do more damage accordingly. Switching into either of those moves can get it killed by the other. Furthermore, a Tera Dark Knock Off does more like 67%-79%. Glowking is not really a safe switch in.
Basically, not much can switch into this mon and nothing unboosted can outspeed it. I feel like it is less obvious at the moment because it isn't a mon that is going to single handedly sweep an opposing team. The pressure on the teambuilder is likely very high, though.