My main blocker for using simplelogin features was that it would make me dependant on proton. But I see this in the new features :

custom domains for aliases

Does this mean that theoricaly if I move away from Proton I will be able to easily migrate all these adresses?

Although this feature looses a big part of its relevance if you are using you own domain as it is a big identifier alone.

Yes this is excellent news and should have been like this from the start!