I would expect it to be the opposite (especially for a linux file system): you want to test with as wide a variety of versions as possible, because that what you're going to encounter in the real world. If your code only works in a limited set of cases, what happens when you mount the file system on a different system, is the file system going to be trashed?

These are all compile time dependencies in this case. You're not going to encounter a variety of versions - that's exactly what the strict version bounds are doing. The app will be built with the tested version and everyone gets the same results.

There's at least 3 different versions that can happen (the filesystem version, the kernel version (including stable backports), and the tooling version). I'm presuming udev being a dependency means bcachefs links to that (which will be different on different OS versions).

Those are completely different things than the dependencies you can make a choice about. You have to handle different filesystem/kernel versions regardless - you don't get a choice here.