To my knowledge glib FAM file monitoring is required to have file monitoring for networked filesystem, as inotify doesn't work on those. So essentially we are forcing that support off for Linux since bug 413403.
Of course as I haven't noticed before and I don't remember seeing any bugs about it, maybe no-one cares about it. And I only noticed while working on glib, not because I need the support, as file monitoring seems to work fine in case of GIO network mounts (I think FAM is about NFS natively mounted stuff, etc).
But even Fedora ships this module still in a separate glib2-fam package, and I believe that's why. Meson build for 2.54 doesn't support building the fam gio module, but the support was added to meson.build recently even, signalling it's still useful - https://gitlab.gnome.org/GNOME/glib/merge_requests/71
We should review the claim of "Enabling USE="fam" on Linux for glib-2 downgrades it from direct inotify support to proxied inotify support through fam/gamin's unnecessarily" from bug 413403 and enable the support; probably should not have it be default enabled via some profile settings though..