I use Proton aliases everywhere...Well not everywhere, there are indeed quite some places that don't accept a passmail.net address... So I can imagine this becoming a useless feature, at least on some sites.

Btw I only use these aliases for sites where I don't mind loosing the login, otherwise it would the mother of all lock-ins... Would have been nice if I could opt for aliases on my own (secondary?) domain... At least then I could still move them (using wildcards or some exported list).