Your proposal requires at least the status quo (aliased email address at signup), but the problem is it also requires cooperation of the receiver's email provider (e.g. gmail) and the sender's email provider. You are unlikely to ever see any new standard adopted by all major email providers, unless it came through a standards committee.
But your idea is interesting. Perhaps you could lower the cost of adoption by replacing the dependency on TLS/SSL with some sort of pgp signing scheme. This way all the "protocol" happens within the message body, so email providers do not need to adopt a new standard. As long as at least one website and one user implement the protocol, it can work without any cooperation from third parties.
As an aside, it would also be nice if password managers included functionality around generating temporary/isolated email addresses.
It’s not a comment. Some mail services allow you to have aliases in the form of firstname.lastname@example.org but that doesn’t mean all do. If it were a comment the address +@example.com would be invalid but it’s not. Comments in email addresses are written in parentheses like username(i'm_a_comment)@example.com.