That's probably wise. I wish there was a way to add metadata to the subkeys. I want to have one set of subkeys for Proton Mail and another set for absolute privacy. I want to mark them as "leaked" keys somehow. Not quite revoked but close.
I read the OpenPGP standard and it seems to have some kind of "notation" packets. Seems to be somewhat related to metadata but I can't figure out how it works or even what its purpose is and it looks like nothing ever uses that anyway.
Of course you are right, if majority of individuals were informed and if protonmail was proactive in informing their users about short commings. The problem is that most users are not informed and they think that protonmail is the bee's knees of email privacy and security, while protonmail only promotes that myth.
