Came here looking for a solution to the same issue. The other thing to add here is that it will occur if the mic becomes disconnected. Even if it is then reconnected, the infinite notifications continue.
Most likely you are running PulseAudio (practically all even remotely modern Linux distributions do this, unless you've specifically decided to run ALSA only etc.). One can create a dummy input (audio source). Run:
pacmd load-module module-null-source
You don't need to run this as root (the regular user has their own pulseaudio daemon on most systems).
Zoom stops nagging now, as it finds "a mic".
However, there are actually three issues here: No situation should create infinite notifications. Moreover, they should not steal focus!
Zoom should use the desktops built-in notifications instead of insisting using it's own notifications at least as an option. The user probably wants to have notifications in one place, not several different places.
Also, this is a regression: previously, Zoom did not send infinite notifications.