libappindicator from Canonical hasn't seen a release since 2012. There's a fork of it, libayatana, which is maintained and should provide ABI compatibility with the old libappindicator (requires proper testing).
Given the stale development of libappindicator, I think it's wise to try to replace it with libayatana. This task is not a package request but a discussion about the feasibility and the usefulness of the switch.
Given @JoshStrobl's approval, let's continue with a list of packages that require libappindicator:
- polychromatic (although it fails to start with Python 3.9)
- steam (multilib)
And a list of packages that require libindicator:
List of packages that require indicator-application:
- mate-indicator-applet (turns out it's not needed)