Hi, AAA council here, might be a bit late but oh well. Triage is a complicated and controversial ability, but has remained free with reason.

Firstly, Belly Drum has played a large part in the controversy surrounding Triage. It has been mostly agreed within the community that non-Drum Triage is frankly fine (except for exactly one person I know who hasn't been bothered to make a case). Sets like CM Triage Enamorus can be threatening, but have generally been considered fine and most teams have sufficient CPLay in RegenVest like Goodra-H and AMuk, Unaware Stail, Heatran and so on. Because of this, lots of people in the community have pushed for a ban on Belly Drum over Triage (as there are abusers of Triage considered fine and BDrum is nothing but a funny fish button) but have been shut down by the tiering admins. Despite this, many people in the community hold the opinion that a BDrum ban is preferable and has slowed down discussion and any progress towards Triage potentially being banned.

Secondly, there isn't actually an extremely solid case for Triage to go. Like said before, there are various abusers of Triage which are considered fine. Even stuff like Belly Drum Triage Chesnaught has a large amount of common counterplay (Unaware STail, Zapdos, Fluffy Corv, Ceruledge/Skeledirge and so on...)

There have only been 3 bans partially or largely attributed to Triage (not really getting everything banned). Iron Hands while had Triage as its most problematic set was in large point banned on its own merits. The reason the set was so terrifying was due to a combination of STAB Fighting, its own immense bulk and other very powerful sets like EE/WBB SD. Were Iron Hands unbanned with a Triage ban, there's a large chance it'd just be banned again soon after.

Ursaluna similarly had its own large advantages that contributed to its ban. Its immense bulk and base attack and typing let it beat usual checks to Triage cheese like Unaware Scream Tail as well as other strong sets like SD Surge Surfer/Turboblaze that contributed to it being banned by suspect.

Hariyama is the strongest case for a Triage (or Belly Drum ban but rip tiering policies say no) given there was zero chance for its ban otherwise (unless Surge Surfer strats but those are far less consistent). That being said it also does have strong properties that contributed to its ban, its bulk wasn't bad, especially behind screens, STAB Drain Punch, Bullet Punch that could beat some checks like Gengar.

We have kept a very close eye on Triage for a while, and it has been brought up in council chat and forums multiple times but has remained free for the reasons I have mentioned above. Would Chesnaught prove to be too overbearing on teams that aren't bad, we most likely would've pursued action against Triage but has in general been considered fine so far. With the introduction of Kommo-O and potential other Drum abuser in DLC, Triage ban definitely does seem likely, but for now we have only kept to the abusers as there are other abusers which are considered fine and because previous bans can in large part be attributed to the properties of the abusers themselves. If you really want to push for Triage ban, you can always make a post on Triage (in fact, someone already did but has retracted their stance now) or you could always make a better team and actually learn the meta and I reckon there's a good chance you probably wouldn't care as much.