Hi Daniel,
Although I agree that we should support tray icons out-of-the-box, I'd rather do this in a way that would be upstreamable so that we don't wind up requiring a shell extension to make this work. AppIndicator has been rejected from GNOME due to serious technical problems [1], so that particular approach seems to be a dead end and probably not useful to focus on. And adding support for AppIndicator now would likely pose backwards-compatibility issues in the future, given that any upstream implementation of tray icons is likely to be incompatible.
The GNOME design team has previously expressed willingness to explore designs for tray icons, and I think GNOME community has a rough consensus that some form of tray icons would be desirable (see the most recent discussion at [2]), but I don't think there are any designs yet. I think we're a bit stuck on coordination problems right now: nobody is quite sure what this would look like, and nobody is working on it. So I think the place to start would be to take this upstream to the GNOME community and try to work out something that could be adopted everywhere, rather than as a shell extension.
Thanks for your interest,
Michael
[1] https://gitlab.gnome.org/GNOME/gnome-shell/issues/1014#note_568908 [2] https://mail.gnome.org/archives/desktop-devel-list/2019-March/msg00020.html