Discord, in its release 0.0.18 has officially updated the dependencies that could support both libappindicator1 and libayatana-appindicator1 by adding both as recommendations (precedence to the former though) in the control file as:
Recommends: libappindicator1 | libayatana-appindicator1
It is good seeing it finally fixed in the official release after four+ such Debian 11 incompatible releases of the app.
Old details
From Debian's Bullseye release, the libappindicator package is depreciated and replaced with libayatana-appindicator.
But, in discord installation packages (including the latest 0.0.17), it asks for libappindicator1 hence the installation breaks in bullseye.
The files in this repo have that dependency replaced with libayatana-appindicator1 after which installation succeeds and the app works fine.