POV: Your phone isn't supported by custom ROMs

https://feddit.de/post/2603124

POV: Your phone isn't supported by custom ROMs - Feddit

Can someone explain what this means?
Yea Telegram FOSS always shows a notification pinged at the top, but Signal never has had to do something like that for me, I just let it run in the background and notifications work flawlessly
For me incoming messages only activated the screen’s backlight without making a sound until I pinned the highest priority notification to the top.
Maybe because I’m using the Signal Foss fork.
Third app that needs this is Threema.