My grievances with Proton

So you are using plus aliasing, this explains why you say you have unlimited aliases with proton mail. Thing is plus mailing is not “real” aliasing, plus aliases are not registered anywhere they merely route to the email you have “plussed”. Other thing is protons free plan supports unlimited plus aliasing so provided all your plus aliases are using your main email they will continue to work when you down grade. What won’t continue to work when you downgrade is the extra email addresses which you get with the Mail Plus plan. Fair enough I think.