If an opponent doesn’t use a Shell Smash User, that doesn’t make Unaware useless. What about PHeal (Quiver Dance, Shift Gear) sweepers, Unburden Belly Drum, and (Simple) Tail Glow can be overwhelming if used on something self-Imposterproof like Gengar-Mega. Also random bonuses like the opponent using Download on Red Orb Groudon, and if you have a set up mon and the foe used Spectral Thief / Heart Swap on you.Unaware, as I have stated previously, is a fairly poor check. The ability grants no extra buff to survival during normal gameplay, which makes it nearly impossible for an unaware mon to keep up with the chip damage of u-turn, rocks, etc. against a smash team. Mold moves further contribute to this problem. Most significant of all, by running unaware you have committed to filling 1/6 of your team with a dead weight in all non smash battles.
Back when Pdon was a thing, it was a quite common to run fur coat Giratina on nearly every team to stop it. FC Tina is overall a far better mon in every situation aside from against smash. It had good utility against all physical breakers (photon wasn’t introduced yet), including things like adapt chomp and adapt ttar. Despite FC Tina being a decent mon, the community decided it didn’t want to have to run it on every team. (So I’m not sure why running a subpar mon on every team is acceptable)
You know what else makes a decent mon? Prankster. Show me a single balanced high Ladder team that does not utilize prankster. I don’t know what to say about this, it seems like the actual definition of meta centralization to me. To be honest I’m quite tired of using prankster on every team I make. It has great defensive utility sure, but I personally don’t want to run the same mon over and over again all the time, however good it might be. The current presence of smash makes teams without prankster or unaware very weak to smash. We are in a meta that nearly forces you to run prankster. I personally have spectated and lost many games where prank registeel or prank ZygardeC dies and the entire rest of the team falls soon after to smash. Not to mention that haze does not constitute a check because it has no way of threatening the opposing mon. It is incredibly passive and relies on the smashing mon not having coverage that kills the hazer (close combat for regi and ice hammer for ZygardeC and to a lesser extent Tina)
So why is it that we are forced to use imperfect solutions like unaware and prankster? Because the community has decided (so far) that it is more worthwhile to keep moves like tail glow, belly drum, and smash than it is to avoid meta centralization and the promotion of otherwise bad Pokemon (talking about unaware here not prank).
By having Unaware Pokémon with resists to Moldy Moves like Gyarados-Mega Versus say Unburden Dusk-Mane, where a Spectral Thief 1HKOs Dusk-Mane, you are better able to punish an opponent, whereas Haze lets them hit you on the turn you Haze, and you have to predict if they will switch out (letting you then switch out), or if they outpredict your switch out of the Hazer first, then they can set up again.
Stealth Rocks and U-Turn chip can be handled with Leftovers (such as a Steel type that resists Stealth Rock - negating Stealth Rocks damage, is resistant to Sunsteel Strike and Photon Geyser, and resistant to U-Turn).
Haze doesn’t stop faster priority either. Think of Belly Drum Triage Heracross-Mega as an example. Drain Punch KOs Prankster Registeel before it can Haze, Leech Life KOs Zygarde-Complete with minor chip
+6 252+ Atk Heracross-Mega Leech Life vs. 252 HP / 252 Def Zygarde-Complete: 517-610 (81.2 - 95.9%) -- 18.8% chance to OHKO after Stealth Rock and Leftovers recovery
Compare that to an Unaware mon, as Xanderpitz pointed out, Yveltal which can come in without fear, even with Stealth Rocks, and use Oblivion Wing to 1HKO and heal from that as well, or Spectral for a KO and a boost.
Prankster Haze resets, while Unaware users can reset with Spectral Thief while punishing the foe into a KO range.
While the set I mentioned is niche, we can also look at Extreme Speeders like Refrigerate MMX which, if it has Shell Smash, can threaten Prankster Zygarde-Complete and Giratina.
Haze is not reliable, as even with Strength Sap, you have things like mixed or Special sweepers that can bypass that (and in MMX’s case, the Photon Geyser switches to Special Attack, and he can attack off of that +2 SpA from Shell Smash).
I think you are underselling Unaware. The only thing that makes Unaware less common is finding something that can resist / is immune to enough of the “right” moves so that means at least 1/3 Moldy Moves, and common coverage moves.
I don’t suggest everyone run Unaware, I do, however, suggest everyone consider Unaware based on what the other 5 members of your team are naturally weak to, and select a specific Unaware user based off of that - as / if needed.
I get that Unaware has 0 purpose if we remove all forms of set up, but you only mentioned removing some set ups, so if the other sweepers that use Quiver Dance, Shift Gear, etc. will still heavily dent our Prankster users (Xerneas to Giratina, and to some extent Zygarde-Complete), Kyogre-Primal with Ice Beam, Moongeist Beam, Quiver Dance, etc. can also threaten them, and Slaking / Regigigas can use Facade for neutral hits on other Prankster users. I just view Unaware as too good to be “deadweight” in any matchup vs a Non-Smash team.
Also, consider things like Tail Glow Doom Desire Dialga. It can take advantage of Haze going first, use Tail Glow on the turn that Doom Desire lands (at the end of the turn), effectively bypassing Haze altogether, whereas Unaware users like Gyarados-Mega, can resist and not get impacted by the the Tail Glow altogether.
I think Unaware has a solid purpose, even if Shell Smash is gone. Removing one set-up, without removing all of them keeps Unaware’s viability alive.