I don't think Pdon and Mdia are really all that problematic, but its more an argument of what "fun" means which isnt really a debate because its subjective, so I'm happy to agree to disagree on what is and isn't "balanced" with regard to fun.
I think MDia absolutely needs to go because it's nigh fucking unwallable, and I've seen a few players bring up how much of a pain in the ass defensive PDon(like LeviDon) is to deal with, so I think it's very reasonable to want both MDia and PDon gone.
a lot of the council decisions in gen 9 have been keeping legacy things banned, or being the fun police (as a long time liquid ooze enjoyer, i feel this especially).
This kinda isn't the case though. The "legacy bans" are all things that absolutely should stay locked up, and Liquid Ooze is deeply fucking bullshit when a lot of defensive mons rely on sap. You mentioned the "not knowing if Bliss/Chans is Imp" thing, and Ooze kicks that up to 2000. Not only do you not know if any of their mons are Ooze until you either Imposter them or die to the Ooze, you also don't know if they're being switched in on you or not every time you try to sap. You aren't even safe if your sap mon would normally beat their Ooze mon, because you're one predict from your opponent away from being out a defensive mon. Fuck Ooze, council made the right decision with that, that's not a "fun police" thing, that's a meta health thing.
That isn't to say that most decisions to ban problematic stuff are bad or the meta is ruined, but it feels like there is now a "right" and "wrong" way to like hackmons, which has made the meta both rock-paper-scissors-lizard-spock and not at the same time. I've been running a pretty unique team for a few weeks now at just under 1500, and the meta just feels like you cant have OK counterplay to everything without losing on the spot to a few problematic mons, or you have a game plan that doesn't beat every set.
I don't get what you mean by your complaints about "right" and "wrong" way to enjoy hackmons, as unless you're using absolutely filthy degen MU fish stuff, none of the bans really interfere with that? As for the RPS nature, that is kinda how every stage of Gen 9 has been since I started playing, but it's only gotten more pronounced recently, in large part due to the PDon and MDia unbans. Imposter is actually one of the better ways to have a decent MU into a lot of teams, so if you are complaining about the meta feeling too much like you can't check everything, why do you also want to remove one of the best ways to check a lot of things?
I'm not claiming that I'm The Guy for making tiering decisions or that everyone is wrong, but I personally am kind of tired about how much normalize, imposter, salt cure, covert cloak, etc are treated as cornerstones of the meta and people are (somewhat rightfully) shamed for complaining about them, but other unique stuff that is powerful without just being objectively big stats (neutralizing gas, gen 9 libero/protean, shedninja in current gen, pheal with no sub or broken setup) is treated like it's overwhelmingly strong to the point that its banned. The format was literally just shaken up in huge ways with the freeing of pdon and mdiance, which has felt like a really strong shift in the right direction, even if they eventually get banned again.
Imposter is treated as a cornerstone because it's one of the biggest reasons for the tier being playable, and removing it would require half the tier to get banned, at least.
As for Normalize, it's kinda not all that scary or important or banworthy, I'm gonna keep it real with you. It fails into 90-95% of good teams unless you dedicate your entire team to removing the plethora of Normalize checks, and even then, it's just kinda mid. MBounce, Plates, GaG, Arceus, Rev Dance, DDesire+Switching, -Atespeed(Like MDia), etc. all fuck over Norm mons pretty hard, so there really isn't a need to ban Normalize, because it just gets countered by a lot of stuff that's already good and on most teams anyways.
Covert Cloak there's no reason to ban, I have no idea why you're up in arms about Cloak of all things, but it is not banworthy.
Salt Cure and Mortal Spin are both contentious, and they certainly aren't treated as cornerstones of the meta. There's quite a few people that want to see one or both get the boot, and I can see some of their arguments.
As for the banned stuff, that shit absolutely deserves to stay banned. NGas is turbo fucking hell and warps the meta into a near-unrecognizable state, forcing a good 80-90% of pokemon to run Ability Shield or do nothing. NGas being allowed single-handedly almost completely kills RegenVests being viable, FurScales mons are forced to run AShield, and run the risk of just getting Knocked and suddenly being unable to wall anything, MBounce has the same issue, Imposter gets fucking ruined, etc.
Do Not Unban NGas.
Protean/Libero deserve their bans because on Choiced sets, it's effectively not been nerfed in the slightest, and on non-choiced sets, it leads to the issue of "well I would use my defensive mon to check them, but I can't because they get a free Band/Specs on coverage that will kill me." Oh, also Monotype mons like Deoxys can hide the fact that they're Protean until they click a non-STAB move and blow you up.
Shedinja is a fucking bastard that would enable insanely degenerate bullshit. You could run incredibly unimproofed shit that just blows up everything, and then just use Shedinja as the improof. You don't even need to use Sunsteel/Moongeist to ignore FurScales, but if you do, you can just use AShield Shedinja, especially with the amount of potential -Ate spin users, hard to block spinners, etc. As for the whole "oh but scure/mspin/etc" issue, scouting can relegate that issue to being a minor footnote, and suddenly you have to either desperately keep all your things that can actually hit Shed alive, or make all 6 of your mons hit Shed.
PHeal is incredibly fucking degenerate because there's still VDance, and there's no Spec Thief or Core Enforcer to keep it in check. PHeal will win against just about everything long-term, and can be slapped on most mons and turn them into a game-ending sweeper. Compound this with the difficulty of beating it with Imposter, and you get a problem.