What about Simple? While I don’t support banning Belly Drum, I would mention that Shell Smash, and Tail Glow especially, benefit from Simple and I think if you are going to argue against those moves, that ability is crucial.Hey it's been a while since I posted here but I want to talk about (as my guy Chessking is already doing) Shell Smash, Tail Glow, and Belly Drum.
I want to start off by saying that I created an HO team in under 5 minutes that abused all of these starts except drum and it went on to win very consistently even in the hands of new players. I really have no experience with HO so I have no doubt that my team is poorly built but the fact that I can give a new player a team I didn't take any time building with 6 mew twos and that player can go on and win against high ladder is a little sketchy in my opinion.
The problem with smash/glow/drum are a few things
1) pokemon are bulkier now with full evs. It used to be that you could just spectral away and the opponents offensive mon would be dead in a few hits. This is no longer the case. Mons like mega ray take almost nothing from spectral and they have ample opportunity to kill your wall with whatever coverage they happen to have. Also fake speed used to kill everything in gen 6 and fake speed doesn't even exist anymore because of dazzling and where it does exist it often can't kill setup mons
2) Dazzling exists. Rip turvy. Rip Espeed. Rip Encore. Pretty much my whole entire argument for why it was ok in gen 6 but not now. if someone were to suggest banning dqm instead of boosting moves that would be fine as well.
3) Mold moves mean shed doesn't work anymore. Also unaware. Unaware was still eh in gen 6 too though because its a lot harder than you think to keep a mon without prankster or regen at full health the whole battle. Chip damage, entry hazards and mold moves all come together and make unaware a pretty bad answer most of the time.
The problem with the 3 moves that I mentioned and not something like swords dance or shift gear or quiver is that none of those moves can win a battle in a single turn. What makes smash, glow and drum uncompetitive is that they can sweep entire teams with very little notice and are extremely difficult to prepare for. The best check to most setup mons is scarf imposter which 1) requires a sacrifice to get in. 2) literally depends on your opponents set lol. Chansey proofing exists. If they do have a proof they will continue destroying ur whole team until you are dead and sad.
It may not be as frequently used as Unburden + Belly Drum or even Unburden + Shell Smash; but with White Herb, it can make Shell Smash Powerful, and make Tail Glow a Belly Drum SpA Boost without drawbacks.
Would Simple make Swords Dance, Quiver Dance, or Shift Gear too strong? Dark-types like Tyranitar-Mega can resist Priority from -Ate Rayquaza-Mega, and variations of Triage Rayquaza with Dragon coverage over Giga Drain, and more importantly, as you mentioned - Prankster Topsy Turvey, without Dazzling.
Simple can also help you bypass Unaware with moves like Power Trip or Stored Power.
I am not advocating for people to use Simple, just that if you feel Quiver Dance, Swords Dance, and Shift Gear are not overpowering, would Simple change your mind? Or is the ability slot cost negating the stat buff advantage, over say another ability like Dazzling?
Should Simple be banned? If people ban Tail Glow, the next best thing would be Simple Quiver Dance, which is in many cases much better.
Lastly, Simple for Tail Glow, Quiver Dance, Shift Gear, etc. won’t require an item slot like Shell Smash or Unburden sets might. With Life Orb and other helpful items like Focus Sash, that could more than make up for not having Dazzling on a HO sweeper.