I am in the same situation and this issue makes Unity very difficult to use for installations as all I can do is confirm or deny that the correct number of Displays are present at the start of the application, not if one is connected or disconnected during runtime.

There is also, somewhat annoyingly, an undocumented event that sounds like it would do exactly what I want, if it was ever raised. `Display.onDisplaysUpdated`.

Closing bugs as "won't fix" and considering working on that only if enough customers complain must be a joke. Please fix this issue.

Having an accurate representation of the connected displays for both Windows and Android is important for a number of my projects. Please consider fixing this bug

But why does the Scripting API say "In a built application, Unity automatically updates this list when you connect or disconnect a display." if you say that's not a bug and you won't fix it?
https://docs.unity3d.com/ScriptReference/Display-displays.html

I too would need this on desktop builds. I am currently working on a project where one or more pojrectors are connected to the PC to display various information of the program within the room.
The people interacting with that system are probably not reliable enough to always turn on the projectors beforehand. Thus I would like to implement a bootup scene which checks how many displays are currently connected, tell the user to connect/turn on the projector if needed and then automatically progress to the correct scnene. But since there seems to be absolutely no way to detect if a new display has been connected, this doesn't work either.

What am I supposed to do in this kind of situation?