That's not my understanding - could you elucidate why specifically, particularly with reference to part of the act if you have access to it, you believe that to be the case?
Here is Itch.io's statement:
We have “deindexed” all adult NSFW content from our browse and search pages. We understand this action is sudden and disruptive, and we are truly sorry for the frustration and confusion caused by...
itch.io
It is incredibly clear. There is no question. The UK Online Safety Act isn't even referred to, and further, you still don't need any kind of age verification to purchase or download erotic material on Itch.io (or I sure don't, in the UK, with a UK IP address), so if that was the case, they're clearly non-compliant!
I mean, your demand here is that what, Itch.io and Steam explicitly list every single thing this plausibly might be because of but isn't? It's not enough for them to offer a positive statement of why, they have to also say why not?
Note Steam also doesn't require age verification, so again, if they're complying with the UK Online Safety Act, well no they aren't lol.