This is not an issue specific to Linux, Windows installations can be downgraded as well. As for the mechanism, UEFI devices, alongside key updates, also get revocation list updates.
With Open Camera, my device (and seemingly many others) have phantom cameras with IDs that crash Android's camera server if accessed. When that happens I have to restart my phone. Open Camera does not have a way to blacklist those. There are several issues open about this on the issue tracker but they have been open since 2020. This is not an easy problem to fix (LineageOS uses a manually populated key called "config_ignoredAuxCameraIds" in device trees to solve this for their Aperture camera app) but at least an option to filter those out manually if I know what I'm doing would be nice.
The other two prominent open source camera apps are Fossify Camera and PhotonCamera. Fossify Camera does not support multiple lenses yet. PhotonCamera is nice because it does image processing and handles my camera lenses correctly but its UX is janky (on my device, with default settings, taking a photo takes 7-8 seconds and quitting the app before the process is complete loses the image), it's not on F-Droid and it doesn't automatically switch between lenses with zoom changes. There's also FreeDcam but I'm not a professional photographer and I'm certainly not going to buy a color calibration reference card that costs more than a hundred dollars.
It sucks that on my phone with /e/OS, instead of using a FOSS camera app, I resort to using Pixel's camera app with internet permission disabled to be able to take advantage of my hardware.
I've been using /e/OS on my phone for a few months now. It comes with built-in DAVx5. Their hosted offering is just another provider and using your own cloud provider is the same process as using theirs. Even their device locator integration is independent from their hosted offering. Of all the Android distributions I've used, it's the one that does providing a hosted service best and it was honestly refreshing.
I have a TicWatch Pro (catfish) running AsteroidOS nightly. If I turn Wi-Fi on, (understandably) its battery drains fast and it gets kinda hot so I leave it off. (I don't know if this is the case for other AsteroidOS watches.) I personally wouldn't recommend it for this specific task even though it's great for anything else.