Imagine if HTTP explicitly documented as part of the standard that the "User-Agent" header, when provided, must include a tamper-proof cryptographic signature that allows to securely identify which browser version the user has (with a consortium maintaining a list of registered browsers) so that websites can reject access if you are not using a browser they trust or they don't like that your browser is too old...

Considering how much abuse there has been with the User-Agent header... I can't imagine it being a good idea to include such a thing in the standard.

If on creation HTTP had included things like that, then I think Gopher and other competitors at the time might not have been a bad option, honestly.