user@domain is specifying a user at a domain, it is not specifying any protocol. The fact that people see it and think it is an email address is unfortunate, because this format of specifying a user at a domain was not originally intended to be used exclusively by email.
An email address URI is "mailto:user@domain"
We could use a URI something like "nostr:nip05:user@domain". I don't like the verbosity though.
But I don't like @user@domain. There is no way to tell that this is a nostr thing at all. Could be a mastodon address. Could be a bluesky address. Might be an email address with a typo.