There’s an API called “client hits” that’s replacing user-agent. Some of the hints will require the user to provide permission for the site to use them, since they could be used for fingerprinting.
Major browsers (Chrome and I thibk Firefox) are freezing the user-agent. The only thing that’ll be changing in user agents is the major browser version. Other parts including platform will be static. Chrome on Windows will always report itself as Windows 10 for example. www.chromium.org/updates/ua-reduction/