Whether being able to change your type to one of the 15 or so different ones on a whim is competetive or not from a macro perspective is part of an arguement that has the right to be made and argued against, but I think everyone can agree that, regardless, it is very uncompetetive on a significant amount of pokemon that can abuse it in a much more noteworthy way than the vast majority of other pokemon. So then I think the question becomes if you have a large number of pokemon that need to be banned because their ability to abuse a game mechanic, would it not be more reasonable to just ban the game mechanic that is causing all of them to be out of control.
I wrote this in the metagame discussion thread, I want to discuss why I don't think half-measure restrictions will ultimately be enough to fix the problem, and will lead to same issue I bolded above.
Showing Tera type at Team Preview
While useful for actual in-game piloting, it does not solve the problem of the stress pokemon that can abuse Tera to it's extreme put on the builder. We saw it with Palafin, where Tera Water was a catch all for most matchups and nuke any potential Jet Punch resisting offensive check, but Tera Steel could be use to get around it's one solid defensive answer, Amoonguss. This isn't just a Palafin problem though, Flying Tera Roaring Moon is going to have to different counterplay than if it uses defensive Tera typings like Fairy or Steel. Volcarona could use a number of different Tera types to resist priorty or hit it's various hypothetical counters with Tera Blast or STAB Giga Drain. I'm wouldn't be surprised if Tera Ghost became a thing as a way to manuever around Blissey. So while it would improve the gameplay experience, I don't think this would be an effective solution in preventing a chunk of bans of pokemon that just abuse the mechanic too well. I'll probably be reiterating this point a lot, but it also doesn't solve the BW gems issue of many games coming down to the guess work of whether you should attack their current or hypothetical typing on crucial turns.
Limiting the amount of Pokemon on any given team that have access to possibly Terastallize during a battle
I think this solution only works if the one pokemon that can Tera is identified at preview as well. I think there's a chance the opportunity cost would lead to more centralized usage what specific pokemon use Tera and what types those are. This seems like it would probably be the most advantageous restriction when it's comes to actually developing a meta around Tera and putting much less strain on the builder. However, it doesn't really change that there are a bunch of pokemon that are just way too good at abusing the mechanic. You're still going to have your Dragonites, Volcaronas, Roaring Moons, etc. whose numerous applications of the mechanic seem too far gone to ever have them exist in a healthy metagame. While this solution would probably have less of a BW gems because there wouldn't be the possibility of having to play that guessing game with multiple pokemon, it's more like Z Moves on steroids. You had a lot of information about Z Moves based on the pokemon, as it was limited to their viable movepool and any move they used within a battle reveals information that can be used to decipher what Z Move is likely. Even if Tera was limited to a single pokemon, it's best abusers could turn into a multitude of different types, gaining coverage they might not otherwise have had or defensively wall a would be check.
Limiting Tera typing to previously existing STAB types
Gives you a lot of info in the battle to work off of, which is a great thing. But there's still a bunch of stuff that have historically relied on their 4x weaknesses (Volcarona, Dragonite, Garchomp, Landorus, potentially Roaring Moon) or their multidude of weaknesses to be kept in check (Toxapex, Dragapult come to mind) and this would additionally give them an 2x STAB bonus or eliminate weaknesses of their choice. I can't seem to articulate a good reason why, but this solution just seems like a weird one to me. Like, if I can change my Dragonite's type to take 35% from your move instead of 70%, does that fundamentally change what is a check or counter to a big enough degree than if I changed the typing to take 17% or 9%? So while I think this would definitely limit the amount of pokemon that end up needing to be banned because of their ability to abuse Tera, I think the question becomes is that number small enough to warrant using a handshake half measure rule simply to maintain a generation's gimmick. I also think we should keep in mind that it's probably best to think of solution that will apply to all pokemon that could be coming to game, and not just the limited ones that we have in the pool right now.
Banning Tera Blast
Tera Blast is mostly just okay. Obviously it does add a lot of randomness in that any pokemon can have any coverage on both the physical and special side, but it's mediocre base power keeps pretty well in check. If this was actually the crux of the problem it would be an easy choice to just get rid of it. Whether it's an offensive or defensive pokemon, changing your typing to avoid being KOd is what actually makes the mechanic so powerful.
No Tiering Action
Plz do something