I have somewhere on the order of 80 accounts on as many websites registered to Protonmail accounts, and I’ve never once had a website turn me away for the @protonmail.com domain. I exclusively use that domain. I can only remember one specific instance where they had downtime, too - overall they’ve been very reliable.

I did once have some of my protonmail accounts closed for suspicious activity, multiple years ago - but in their defense I was being pretty suspicious. I emailed support from my main account, explained the situation, and they unlocked the accounts for me - never had an issue since.

There definitely are sites that block or restrict accounts using ProtonMail addresses. Obviously social media sites will be more likely to force an account to provide a phone number if one is using a proton mail address compared to for instance, a GMail address. There has been one site I have seen block/restrict ProtonMail outright, and that is Lenovo. It is not possible to actually make purchases from Lenovo directly if the account's email is a ProtonMail address. Too be fair though, that is the only site I have seen any explicit account blocks/restrictions.