Sadly, yes. A Proton team member on Reddit confirmed it a couple years back:
Folder/label names are visible to the server (for filters and other reasons) as are email metadata. Message and attachment contents are encrypted and not visible to the server.
They implemented their own way of doing this. They don‘t use PGP. I don‘t know how it exactly works, but it‘s open source, so you can look at it yourself.
Is that true? They don't encrypt labels on proton?
Sadly, yes. A Proton team member on Reddit confirmed it a couple years back:
https://old.reddit.com/r/ProtonMail/comments/hxomoy/are_mail_folderstags_encrypted_or_private/fzc6wry/
Do note that this is ONLY true for Mail. Calendar, and Drive does encrypt folders/label names.
Subject lines are also not encrypted, unlike Tutanota.
How would that work? Email doesn't have a subject line encryption standard.
They implemented their own way of doing this. They don‘t use PGP. I don‘t know how it exactly works, but it‘s open source, so you can look at it yourself.
The problem is it doesn't work unless everyone does it the same way. Pgp isn't perfect but at least its a standard.