AFAIU, the plan from Google is to freeze the User-Agent header value (with some support from other vendors) and introduce new client hint headers in their place, along with a new JS API that can be used instead of navigator.userAgent in order to target specific browser versions etc.

Intent to Deprecate and Freeze: The User-Agent string: https://groups.google.com/a/chromium.org/forum/m/#!msg/blink-dev/-2JIRNMWJ7s/yHe4tQNLCgAJ

Intent to Ship: Client Hints infrastructure and UA Client Hints: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/A4wxFpvqUfA/g7iccl9ICgAJ

Spec on JS API: https://wicg.github.io/ua-client-hints/#interface