I am not sure about opting out, but Conrad IST shouldn't be one of them as its managed by Hilton. I've had auto at franchised and managed. In IST I've had such auto upgrades.

They could just block the inventory (like the rooms are dirty, broken etc) preventing auto to take the inventory. It's run locally, so properties you could just block them off for 15:00 local time 3 days in the future and it won't allocate inventory. With the way Hilton deals with properties I would say managed are more likely to do stuff like that than franchised. I find franchised play things by the book and on my travels are more likely to be pre-Covid standard than managed.

The key take away is it will never upgrade you to a type where there is just one example, covering most named suites, like Presidential, Royal and if occupancy is 50% or higher in the upgraded types it will skip or not work.

I think its beneficial in hotels that don't upgrade often, as it locks it in and you could complain if they remove it. Properties that were always generous will remain generous. For example Canary Wharf.