Intent to unship: Exposure of 11.x macOS versions in the User-Agent string

[Henri Sivonen]

https://bugzilla.mozilla.org/show_bug.cgi?id=1679929 caps the macOS
version exposed in the User-Agent string to 10.15.

The motivation that set this in motion is the Web compat impact of
sites not expecting to see a version starting with 11. The reason for
not exposing Big Sur as 10.16 is that Safari capped the version and
capping the version has a privacy benefit going forward while the
utility of exposing the macOS version is questionable especially after
Safari capped it.
https://bugs.webkit.org/show_bug.cgi?id=217364

As for why not do even better for privacy and report 10.15 on older
versions of macOS, capping the version is a more prudent change.

Chrome will also cap the version like Safari in its UA string, but
Chrome will expose the real version via Sec-CH-UA-Platform-Version,
which neither Safari nor Firefox supports. (Client Hints in general
and the Sec-CH-UA-* parts in particular are beyond the scope of this
email.)
https://groups.google.com/a/chromium.org/g/blink-dev/c/hAI4QoX6rEo/m/qQNPThr0AAAJ

So far, the use case why Web developers would prefer to distinguish
Safari on Catalina vs. Safari on Big Sur relates to WebP support, but
the issue is moot for both Firefox and Chrome, as Firefox and Chrome
advertise WebP via the Accept header and don't rely on the OS decoder.

--
Henri Sivonen
hsiv...@mozilla.com