I think adding it to desktop-legacy is a good idea: ideally we’d be able to phase out unity7, so ideally any API still in use should be available through one of the other interfaces. It isn’t clear that it is a candidate for desktop though:
- App indicators are not currently supported by upstream GNOME: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/1014
- While they do recognise there is a desire for this feature, they don’t want to adopt Unity 7’s dbusmenu code (and in fairness, we were phasing it out in favour of GMenuModel anyway).
- It’s not clear how confinement friendly the app indicator API is.