Services are slowly cracking down on aliased emails, some already rejecting @passmail.net. A good alternative is having a custom domain, however a quick chech to the MX record will give away that it uses "alias.proton" and can potentially lead to new blocks.

Having the alias server be either merged to the regular proton traffic would be a nuisance for servers to distinguish aliased from actual emails, removing this single check: "if MX contains alias.proton, reject domain"