While it is very much so debatable at which point the threshold of "extreme degree" has been reached, I would say it is unreasonable to expect Cloyster to be denied the opportunity to ever set up a Shell Smash, in a long game where adequate support through Screens, moves like Memento, and even just decent pivoting and positioning can be enough to allow this to happen, after which the game is very much so taken out of the hands of the player unless they have the explicitly adequate type of priority or a faster Choice Scarf Pokemon, and it is also unreasonable to expect all or most teams to have these. I would also say that Bright Powder and Sand Veil remove agency from the player at nearly no cost, meaning the player is always in a state of just hoping their moves will land, with no other level of nuance or strategic play considered. Quick Claw similarly makes players hope that it just doesn't activate.
Lastly I would argue the lack of consistency, viability or tournament results are not solid enough reasoning to keep these. The idea behind these strategies is that they allow a lower skill player to get the upper hand on a higher skill player, so you will not see these be used by high skill players, nor will you see them reach high tournament placements. Nonetheless, tiering policy should always aim towards reducing these kinds of situations at the utmost, and the removal of items and abilities that have no purpose other than reducing skill expression should be a no-brainer.
No offense, but are we even playing the same tier? While screens are somewhat common (though not as much as they used to be), when is the last time we've seen Memento in an actual game? I also think you're severely overstating the ease with which Cloyster can set up, even if we assume screens/Memento are in play. There's plenty of other options to either limit it or straight up prevent it from setting up, and those are most definitely not unique to very few select Pokemon. Besides priority, you also have Knock Off, Thunder Wave/Glare, Roar/Dragon Tail/Whirlwind, Taunt, Wisp/Toxic putting it on a timer (which is really bad considering you need at least 2 hits to kill everything that isn't a team with purely offensive Pokemon, which is surely packing some priority at least), and even more fringe stuff like slow Encore on Clefable. For priority there's Mega Lopunny/Medicham as a conditional check to Cloyster, especially if paired with something that hits it on the setup turn. Conditional because Protect as a 4th move can stop it, but that's quite rare. For less conditional checks there are Bullet Punch MMeta/Scizor, Sucker Punch Bisharp, Priority Thunder Wave from Thundurus, Mach Punch Conkeldurr, as well as stuff like Extremespeed on Dragonite. There's also Sand Rush Excadrill being extremely common and beating everything that isn't King's Rock Ice Shard flinches. Almost none of these kill Cloyster from full, even at -1, but definitely after a hit on the setup turn if you don't give it completely, 100% free setup with no rocks up. Most of these are excellent Pokemon in their own right, and that's only if you actually concede it a free Shell Smash. You'd be surprised at how few actual setup opportunities Cloyster gets, even with screens up. I mean, look at this:
0 SpA Clefable Moonblast vs. -1 0 HP / 0 SpD Cloyster through Light Screen: 137-162 (56.8 - 67.2%) -- guaranteed 2HKO
Even assuming the Clefable has nothing else to stop its sweep with, that's at most 3 sand turns away from dying if you have Stealth Rocks up, to give you another potential out not involving being explicitly faster. If you give Cloyster 100% free setup with no rocks, no plan, no priority, nothing to chip it with, and then lose because it flinches three of your Pokemon... Maybe you shouldn't have given it so much space to do whatever it wants and deserve to lose? Did the worse player really beat the better player if you're giving me free screens, a free Memento, and free setup with my Cloyster, right after I clear the field of hazards? Let's also not forget that this isn't even close to a win, it's
a less than 50% chance to even do anything. Lets also not forget that, especially against fat teams where this is the most relevant (as I will say that this is probably the set with the best odds of breaking Slowbro), King's Rock being explicitly the best set only applies to very few Pokemon. NMI Cloyster knocks out many of the other targets with minimal chip damage. The effect King's Rock in particular has on Cloyster's ability to break through most Pokemon is hugely exaggerated.
+2 252+ Atk Never-Melt Ice Cloyster Icicle Spear (5 hits) vs. 252 HP / 252+ Def Clefable: 380-455 (96.4 - 115.4%) -- approx. 87.5% chance to OHKO
You don't exactly need King's Rock flinches to break through a lot of its "checks", and, again, this only happens with absolutely perfect battle conditions. I'd argue that it's actually very competitive that your opponent's sweeper breaks through your team if you give it absolutely free reign to do as it pleases, personally. Teams rarely need to dedicate space to actual King's Rock Cloyster answers because they're baked into most teams by default, that's just a matter of fact, and if they aren't, they probably are quite weak to Cloyster in general.
I don't see how the lack of results is a bad argument here. Are you arguing that we should ban Dunsparce or Sawsbuck for abusing a team full of paralyzed Pokemon on your opponent's side too well with Headbutt? There's a reason good players usually don't gravitate towards those strategies, and that's because they're unreliable and just not that good. This is no Swagger situation where you're in constant danger of hitting your own Pokemon for 40-50% while your opponent spams Foul Play, or something becoming almost invincible with 1-2 uses of Minimize on already good Pokemon.
This is a Pokemon giving up its item slot to, at best, get a below coinflip chance to luck past some of its checks, given perfect battle conditions and matchup. And, again, most of these checks falter against plenty of other Cloyster sets. Unlike those two strategies, it also has far more counterplay than bad/inferior abilities like Own Tempo and moves that see little to no actual usage, like Aerial Ace, Shock Wave, and Aura Sphere. These avenues of counterplay already exist on a vast, vast majority of teams. I don't see how a Pokemon requiring this much setup is worse than most other "luck" angles in Pokemon, unless you want to argue that we should ban most forms of RNG? In which case, why the hell are you playing Pokemon if you intrinsically hate RNG elements, regardless of how good they are?
I can see more of an argument for Quick Claw, but we should absolutely analyze results for this. We usually ban things because they're broken or uncompetitive, and I don't see how Quick Claw robbing one game a year is anything close to full Swagger teams turning full games into actual slot machines. Quick Claw has counterplay, too, in that you don't always need to outspeed things to beat them. I'm not going to go into detail about this, I can gladly make another post down the line as I do feel rather strongly about this topic, but until there's more (like, a lot more) games of Quick Claw Manaphy being a problem than it sniping Conkeldurr Trick Room in SPL, I see this as little more than a fringe strategy that steals games once in a blue moon. I've seen more games stolen by Mega Metagross getting a 20% proc on Meteor Mash/Zen Headbutt, and it's not particularly close. And believe me, I've probably experimented with Quick Claw in this tier more than most people.
Again, no offense, but your post sounds like a bunch of theoreticals that have little to do with actual ORAS OU games.