The User-Agent header is not mandatory and was never intended to be used by tech companies for denying access or fingerprinting. It was supposed to be used, at the user's discretion, to help with interoperability problems. RFC7231 specifically refers to user-agent masquerading by the user as a useful practice. It explicitly discourages using this header as a means of supposed user identification, e.g., fingerprinting.
Setting your user agent would only be considered hacking by the same people who think the Internet is a series of pipes. The browsers themselves copy each other's user agents for interoperability, so it's far past the point that changing it to look like another agent would be considered devious.
Since it became a dissemination service for public officials. The moment it became illegal for the US President to block people on Twitter, it should have become illegal for Twitter to restrict access to information to the public, for the same reason.
