No, it is not. bcachefs needs to have all the code for error recover in the kernel as it needs to be available when a storage device fails in any of a myriad of ways.

Maintaining a piece of code that needs to run in both user space and the kernel is messy and time consuming. You end up running into issues where dependencies require the porting of gobs of infrastructure from the kernel into userspace. That's easy for some thing, very hard for others. There's a better place to spend those resource: by stabilizing bcachefs in the kernel where it belongs.

Other people have tried and failed at this before, and I'm sure that someone will try the same thing again in the future and relearn the same lesson. I know as business requirements for a former employer resulted in such a beast. Other people thought they could just run their userspace code in the kernel, but they didn't know about limits on kernel stack size, they didn't know about contexts where blocking vs non-blocking behaviour is required or how that interacted with softirqs. Please, just don't do this or advocate for it.