I do not know of a single case in which Valve stepped in when developers/publishers refused/ignored.

There was the Helldivers 2 debacle, where they retroactively wanted you to create a PSN account to continue playing the game. I’ve heard from a few people, who got a refund over it. The automatic system didn’t work though, they had to write with the steam support directly.

This isn’t exactly platform support. But I think, if platform support was/is listed on the store page, you would have a stronger case than Helldivers 2 though, where the third party account was listed on the store page.