Even with the first, Google supports forwarding all mail to an arbitrary email address, which means the "Gmail" becomes a text-file stored on a dovecot server (or other regular mail server) somewhere.

So unless they break delivery (you can forward only some subset of emails) - there's no way for the sender to have any better guarantee than "please delete after reading".

At least the pgp spec has (had?) an "eyes only" flag that, while it didn't guarantee anything, at least meant compliant software would try hard to not leave a plain text copy on the filesystem.