Forgive a bit of ignorance on this as it might be a dumb question, but now that bcachefs is a kernel module and not part of the kernel directly, is it still realistic for people to run bcachefs as their root filesystem? Do you know anyone doing this?

Distros generally build everything they can as modules these days, including filesystems. No reason not too, we've had initramfs since forever; you can't build everything in that anyone might need to boot their machine.

As long as the testing pipelines are in place to make sure the dkms module builds on every distro configuration (a good chunk of that is still manual, but there's a project to improve the test infrastructure) - in practice, no one will notice.

I wouldn't have noticed the DKMS switch on my NixOS laptop if I didn't know it was happening.

Just looking at that factor should be about as realistic as running ZFS (very).

bcachefs was always a module. You don’t want it in your kennel if you are not using it. The difference is that it used to ship in the mainline source code and be built as a module that was already built and on your drive.

If you build bcachefs as a module yourself (via DMKS or directly), it works the same as if you got it with your distro.

If you use bcachefs as root, the danger is booting with a kernel that lacks the module.

I hate that bcachefs is not in the kernel, and my primary distro does not use DKMS. But, if you can get a module built, there is no loss of functionality or performance.