I think the main issue about Quick Claw is that it's an item that can completely invalidate offensive counterplay 20% of the time.
Like in a situation where I see an Ursaluna clicking SD and I have a free switch to my Choice Specs Walkign Wake, I should be able to reasonably expect that My WW should counter this Ursaluna by OHkoing it.
One of the problems with Quick Claw, is that there's no way to scout for it. It's not like a pokemon having a choice scarf, which you could reasonably determine by switching out and seeing that they stayed in. Quick Claw is completely up to chance, and is an item that can ultimately give wins to the worse player, since there is no reliable offensive counterplay to Quick Claw.
You make a fair point about how Claw can not be scouted for, but it can be inferred. Take the example you gave, Ursa has to Swords Dance before in can start OHKO everything that exists. But once you see the SD it is reasonably assumed that it is not scarfed, so it is either Flame Orb Guts, or it is claw. If the Ursa doesn't get burned then you know that the worse case scenario is it having a quick claw.
One can make the argument that the issue isn't scouting the claw, but that once the ursa sets up it can cleanly sweep with the highrolly 0.0064% to wipe out the entire team. I think this presents an opportunity for the meta to shift. If this starts becoming more of a growing problem then teams will start switching from Hard Offense teams to bulkier balance teams.
The reason why I love Quick Claw is because you aren't immediately aware of the item, it has to be inferred by the puzzle pieces that the opponent shows. For example if opponent has a lot of slow mons with no trick room setter, you can safely assume that these mons have items that either bulk them up, or increase their speeds. This inference is something that everyone does, it's just one step further from the Choice Items or Assault Vest.
Along this reasoning is why I believe that Tera should not be banned. An example of this is if I have a King Gambit that sets up an Swords Dance and the opponent switches into a Great Tusk, I can tera Ghost to avoid the Close Combat and can either set up another dance or start clicking Kowtow Kleave. (However I know that this is a much bigger issue because there are 18 different types a mon can tera but you should still have an idea of what is the most ideal tera for a mon but whatever)
This idea of team building is vital to the game to keep it adaptive and fresh. Too many Valiants running around? Tera Steel your fairy weak Sweeper. Your Psychic types are losing to too many dark moves? Run a Colbur berry to surprise them. Ground moves are spammable against your team? Add a flying type to your team. A slow mon can't two hit KO a check? Run Quick Claw to hopefully outspeed it. Are you scared of a particular phyiscal sweeper, run Will-o-wisp. Need speed control? Trick Room or Thunder Wave. These decisions are all put together in team building to help your mons win a match up against what the creator generalizes the meta to be. If there is a glaring weakness to your strategy then you patch that weakness with Tera, item or moves. I want to make a guess that people only started using quick claw because it was a tool that people found to counter the HO teams that have been becoming more popular with Iron Valiant and Enamorous (wasn't the 'abuse' of quick claw showcased in a tournament I don't know) Sure this may be an extreme simplification of how pokemon works, but its what it is at the core
Now for the inevitable comparison to other hax items. Kings Rock is easily abused, so easily abused that it takes no thought to compare it to Skill Link mons or Multi-hit Population bombs. This is different because the 11% flinch chance can easily be increased to 50% a turn by the skill link mons leaving the opponent's turn down to a coin flip. Evasion and Bright Powder are the closest comparison for why it should be banned. Both take initiative out of players and leave the game up to chance. But what's the difference? Bright Powder reduces the opponents moves accuracy, and this is inherently uncompetitive to pokemon, because if you run a 100% accurate earthquake, it should be able to hit 100% of the time instead of the lowered 90% by Bright Powder. However quickclaw changes speed, and changing speeds is already been proven to be allowed by this community (Dragon Dance, Trick Room, Paralysis, etc) Claw is only considered broken because it is an inconsistent way to move first within a priority bracket, however priority moves can still move before a quick claw attacker.
Finally, I want to address an issue that some people don't understand is a big deal with quick claw. IT TAKES THE ITEM SLOT. This is a huge deal that I think people overlook, There are much better items that bulker mons should run if they want to survive, for example they can run leftovers to help live 2 hits, it can run AV to patch up its SpD stat, Or if it wants to hit like a truck it runs band or specs. It could even be guaranteed to be a faster mon if it runs a scarf. This cost makes along side the 80% of do nothing makes the item incredibly worthless. But just like how Choice items lock you into a move as its downsides, that 80% downside has the huge 20% upside of going first.
So this comes down to my finally question, what's hax'd about claw, is it the randomness? If it is the randomness what should the accepted level of randomness be? Should the council ban Stone Edge because it increases my Critical Hit Ratio? Should they ban trick because then the opponent doesn't get to use their specific item? Should they ban Volcanion because Steam Eruption has a higher burn chance? Should Flame Body and Static be removed because what if I want to run all physical attackers and I have a chance to be statused? Should Dire Claw be banned because it's too random? This is the issue I have with those that don't like the idea of Quick Claw, if randomness is uncompetitive then why is everyone playing a different team, with different moves. If we wanted a true test of skill we should create a new ladder where the only team allowed is pre-determined by the council as the best 6 pokemon, with the best stats, with the best moves. This way we can see who can outwit the other with the same team.
TL,DR Quick Claw is an acceptable level of randomness that is in an inherently random game. Quick Claw is being used as a way to adapt to a changing Post-HOME meta game as a way to check/threaten Hard Offense Team that want to outspeed and outdamage the opponent (As quick claw helps you do)