Arch Linux

Not a packaging issue, they were dropped upstream:

https://gitlab.gnome.org/GNOME/adwaita-icon-theme/-/issues/136
https://gitlab.gnome.org/GNOME/adwaita-icon-theme/-/commit/f24f640e44aab45730be9ff64dcf46a0a7639a94

Not sure if xfce was supposed to stop using those icons (marked "legacy"), are any changes needed there, or do we want to package those legacy icons separately?

same here, I am missing some Firefox icons.
I tried: "gtk-update-icon-cache -f -t /path-to-icons"
but nothing changes

Ironically, gtk seems to be taking over a bunch of the icons which were dropped from adwaita-icon-theme: https://gitlab.gnome.org/GNOME/gtk/-/issues/4754
In the gtk3 branch, 142 icon files were added on 2022-03-10: https://gitlab.gnome.org/GNOME/gtk/-/commit/5a0ffbbb4568e39bdf26006e1bf18c1c1d0d597a
That commit didn't make it into a release yet, since the newest gtk3 release was 2022-03-05.

So I guess these are some options that will fix the icons missing from different applications etc.:
* Wait for the next gtk3 release (or build one with the above patch) which will fix the problem automatically if the missing icons are among those (re-)added there.
* Patch upstream software to migrate away from deprecated APIs etc. This fixed the issue for virt-manager (although it might have changed some icons from full-color to "symbolic" – don't remember the exact looks from older versions): https://github.com/virt-manager/virt-manager/issues/361
* Ship the missing icons with the software. Example: GNOME Evolution https://gitlab.gnome.org/GNOME/evolution/-/issues/1698
* Use different icons which are still included in adwaita-icon-theme (or gtk).

I'm not a maintainer, but I guess cherry-picking the gtk3 patch until the next upstream release might be reasonable. The others should probably be tackled upstream. Packaging a legacy icons package like suggested by ghen should also work as an intermediate solution until things have been worked out upstream.

(By the way, pavucontrol is also among the applications missing icons, e.g. audio-card.)

I have the same issue with Cinnamon (5.2.7-2) and v42 of adwaita-icon-theme, loads of icons missing in the menu. Downgraded to v41 to keep it visually in check.

Issue still present with adwaita-icon-theme v43 (and latest gtk3). How should we proceed with this?

Same problem with xfce. I copy adwaita-icon-theme V41 in /local/share/icons. My system is update with V43 but no icon are missing.
Waiting solution... May be it's stupid ?

what's the upgrade path for existing installations? In Settings > Appearance > Icons, I only see "Adwaita and "HighContrast" themes. Are there additional packages to install?

xfce4-settings 4.18.0-1 depends on elementary-icon-theme so it should appear in Settings -> Appearance -> Icons. You do need to select it manually though.