[sudobash1]

I'd say that isn't true. The Linux version number is somewhat arbitrary, and does not denote incompatibility. And from the view of userspace, the Linux ABI is rather backwards compatible.

Also, WSL1 and some BSDs are examples of Linux compatibility.

[baq]

Yes, the driver interface may or may not change, nobody knows where and when. That's a feature of the Linux versioning scheme and driver HAL policy. You can roll your own userspace-Linux-compatible kernel and it's going to be a toy until you develop drivers for it, which won't ever happen.

> Also, WSL1 and some BSDs are examples of Linux compatibility.

Yes, and they suffer from either performance or lack of drivers, hence WSL2 running a real Linux with some dark magic to share hardware and network, sort of. Surely you don't mean this as an argument in favor of rolling your own Linux-compatible kernel?