It’s not just Protonmail.
Blacklists like these aggressively and unapologetically collect all privacy-focused email domains they find, including simple forwarding and tagging services. With more and more sites using these lists to reject or black-hole email addresses, it has become difficult to protect one’s self from spam and cross-site account tracking.
Dear web developers, please don’t use these lists. Well-intended or not, they are privacy and user-hostile.
A web forum is far better in most cases. If you can’t manage to run your own, there are plenty of lemmy servers that will do it for you. Even an email list (with searchable archives) would be better than Discord.
If you have collaborative documents that outgrow the forum format, use a wiki.
If real-time chat is needed, irc or matrix.
A project hosting its community on Discord is a project that won’t get my contributions.