A bit more technical detail:
That the pref approach in the landing wouldn't work for linux distro
builds was clear, at least to me. There are no language-dependent pref
files in gecko.
Now, the distro builds don't ship en-US builds, i.e., the
firefox-l10n.js is empty. Instead of that, they're adding a pref to
switch on matchOS.
Which means, by that assumption, the feature should have been preffed
off for all locales including en-US.
It's unclear how that pref made it into the linux distro builds, if
that's just a weird artifact from the build process they're using, or an
explicit decision of at least some distro maintainers to switch this
feature on with hardcoded English strings.
Axel