There are valid reasons to want to know the specific user agent when working around bugs in old but popular versions, that's why I said a user-agent allowlist for access or advanced functionality was bad. That inevitably breaks any new platforms that haven't been explicitly approved, which is obviously bad. Denying access from or specifically only applying fixes to known bad versions is fine.
Also IMO it's useful as an admin to know what clients your users are using, but I do understand why many would prefer to limit the data shared with the sites you visit.
The abuse of the user agent for tracking and unreasonably blocking browsers - Chrome only websites that work fine when you spoof the user agent - out weighs the usefulness of being able to work around browser specific bugs.
These days old browser versions are for most companies, a problem of the past. IE is well and truly dead, and almost all users have auto updating browsers now.
Websites shouldn't really attempt to fit themselves to the browser, just detect which features are available and if there is some odd browser bug, wait until it's patched if it's affecting a major browser.
Also IMO it's useful as an admin to know what clients your users are using, but I do understand why many would prefer to limit the data shared with the sites you visit.