You are correct that Deo-S has some versatility on its attacking set, but no more than the typical OU offensive Pokemon. Basically, on offensive Deo-S, only the last slot can be changed, as Psycho Boost, Ice Beam, and Superpower are all musts in order for Deo-S to do its revenge killing job. Yeah, it can use HP Fire, Fire Punch, Knock Off, Extremespeed, Rock Slide, or Thunderbolt, but the general answers, such as Aegislash, Mega Mawile, and Slowbro, don't really care about most of its moves, and even if they do, they can still take one super effective move from it and OHKO back, or scout the move it has on the last slot so they can go to something that walls it without a Fire move, such as Scizor or Genesect.
Please correct me if I am wrong, but what do you
need Ice Beam for aside from Dragonite, Mandibuzz, SpD Hippowdon (all of which don't appreciate losing their item)? Psycho Boost wrecks Thundy/offensive Lando/Gliscor, does like 76% min to Garchomp, is stronger vs Mega Zard, and Salamence kind of sucks now so lol at that. Knock Off also covers several things you would use Ice Beam for:
20 Atk Life Orb Deoxys-S Knock Off (97.5 BP) vs. 4 HP / 0 Def Latios: 216-255 (71.5 - 84.4%)
20 Atk Life Orb Deoxys-S Knock Off (97.5 BP) vs. 4 HP / 0 Def Latias: 195-231 (64.5 - 76.4%)
20 Atk Life Orb Deoxys-S Knock Off (97.5 BP) vs. 220 HP / 0 Def Celebi: 177-211 (44.6 - 53.2%)
Yeah it's not as effective against Celebi (still knocks off its item), but it actually outdamages Ice Beam against the Latis which are more common/threatening targets:
248 SpA Life Orb Deoxys-S Ice Beam vs. 4 HP / 0 SpD Latios: 190-226 (62.9 - 74.8%)
248 SpA Life Orb Deoxys-S Ice Beam vs. 4 HP / 0 SpD Latias: 164-195 (54.3 - 64.5%)
Knock Off also has the bonus of hitting enemy Psychics like Espy/Starmie/other Deo which is also worth noting...
Granted having Ice Beam is very important for Dragonite and Mandibuzz (not that Buzz is too hard to check but its worth mentioning I suppose), but you don't necessarily
need it. Arguably you only
need Psycho Boost and Superpower, leaving two slots available to use for HP Fire, Knock Off, Dark Pulse (it does 40% min to Aegis and is consistent damage while covering Gengar and Psychics which is neat) Rock Slide, TBolt, Ice Beam, even SR and/or Taunt if you wanted a hybrid hazard/offensive set. Deo-S has a lot of flexibility so that it can conform to what the team needs.
I guess my point here is that Deo-S is a lot more flexible than what it may seem which is a legitimate problem considering it literally has moves running out of where its nose would be if it had one and you never know what it has or can have until it makes a move. I'm not really arguing one way or another for Deo-S's ban, but it is more menacing than you seem to think it is.
And Aegislash is perfectly viable on HO teams. Checks a ton of stuff, has set up potential with SD + Autotmize + Weakness Policy, is almost unwallable, and has strong priority.
Lol I don't doubt that. Still, a "hard counter" getting Knocked Off for 40% damage minimum and its item being evaporated is a tough price to pay for something that is needed to check a lot of stuff.
EDIT - Oh my bad @ below.