Note: not a dev, this is my partial understanding based on some reading
ProtonVPN 4.13 → 4.14 migrated from GTK3 to GTK4
but AyatanaIndicators’ GTK4 support removed legacy XEmbed fallback, which is what allowed the tray indicator in QubesOS.
GTK4 only supports StatusNotifier, which does not go through QubesOS’s compartimentalization.
See also: RFC: Remove appindicator (system tray) support · Issue #192 · transmission-remote-gtk/transmission-remote-gtk · GitHub, Lark-Gtk | lark-for-linux