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.