Sure, the reasoning for username+secure is that most mail servers are set up to forward that, so if I send a pgp encrypted mail to user+secure@example.com today

Sure, the reasoning for username+secure is that most mail servers are set up to forward that, so if I send a pgp encrypted mail to user+secure@example.com today, user@example.com will get it, and can read it using regular mail+pgp setup. The server could then be augmented to drop unencrypted mail to "+secure" - and mail user agents could handle pgp keys and "+secure" addresses especially. For example split mail delivered to user@ and user+secure@ in a "insecure/postcard/public" view and a "private/personal/letter-in-envelope" view.