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.
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.