Ah, I should have said "from a domain you own or one of their own".

The use case I'm talking about, which is the use of arbitrary domains, not Proton-provided ones and not domains you own and control.

I see that Simple Login provides aliases from its own domains, but not a way to use an arbitrary domain.

Proton's address support overview mentions organizational addresses, but clarifies in the same doc that this is referring to a business plan where that whole organization will be using Proton.

Proton's switching guide discusses forwarding, and it only instructs the user to tell their contacts about the new Proton address, which defeats the purpose of forwarding addresses.

Here is further discussion about the missing functionality.

Meanwhile, Google lets you use up to 99 of your own email addresses from whatever domains they are.