As for you can do in this situation, there are
multiple lists online where you can find other privacy-respecting providers, including Proton itself, of course.
As for this particular measure, without calling anyone out, I strongly disapprove of it! Not only does this limit actual well-meaning users' access to the forum, but with the nature of some discussions, it should be in everyone's interests to support user privacy, and not require the user to figure out what provider isn't banned just to give up and use something like Gmail, which is literally panopticon.
I realise I have zero privilege over moderation and any high-level decision-making, but moving the responsibility of fighting spam (in this case, alternative accounts, etc.) to the email provider is a fool's game. In the long run, it could cause smaller, trustworthy providers who
don't unnecessarily check or collect anything to be on the chopping block as soon as one person starts using it to create multiple accounts.
Sure, Proton is pretty large nowadays, but even then, if I were going to become a new user and only used a Proton address, I wouldn't bother registering.
Edit: Considering the language used on the registration modal, is the "ban" mentioned previously meant to be the "additional verification" procedure?