(In reply to Masatoshi Kimura [:emk] from comment #0)
Chrome announced the timeframe. Safari already froze the user-agent header (except OS version and browser major version).
I'll just note that Firefox has shipped with support for this since FF58 (Bug1333651), although behind a pref. Tor Browser has frozen its UA string (except moving between ESR versions and OS) for most of the last decade (originally via the |general.*.override| prefs).