It is also security theater. 99.9% of the time the other side you are communicating with stores their mails with server-side encryption. If your fancy encrypted e-mails have a "plaintext" mirror, your encryption is useless.

You want to optimize your 99.9% case for convenience (say, use Fastmail), and optimize your 00.1% case for security (manually managed PGP with a secondary anonymous e-mail). It makes no sense to trade away swathes of convenience and security just so you can be lazy with your 00.1% case.

I view Proton Mail as the convenient 99,9% case. It's a very polished service and it seems to offer a somewhat higher security baseline than the other email providers which probably don't even try to do anything encryption related.

The maximum security manual OpenPGP 0.1% case is still absolutely necessary though. No doubt about that. Anyone claiming that Proton Mail solved this doesn't actually understand how OpenPGP works. Not that I would fault them for failing to understand this ludicrously complicated stuff.

I can't deny that possibility. Still, it should be an individual's choice to risk it or not.

It is but if I exchange emails with a Protonmail user I am writing with them like there is no encryption present.

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.