There has to be an ABI in order for loadable kernel modules to exist. That's the part that's infamously unstable.

API stability is not guaranteed, though it's stable enough that the likes of Nvidia can write installers for their proprietary drivers that recompile a bit of interface glue between kernel and blob.

> There has to be an ABI in order for loadable kernel modules to exist

Given a particular install, yes, but it will change independent of kernel version based on e.g. build flags and even conceivably compiler chosen, etc.

> it's stable enough that the likes of Nvidia can write installers for their proprietary drivers that recompile a bit of interface glue between kernel and blob

Well...they certainly can. It also can infuriate me beyond all reason...

The way I've understood it is that proprietary kernel modules contain an opensource part that is included in the kernel which makes a small stable API available which is then used by the proprietary kernel modules.

Making a stable API that covers the entire kernel would be a monumental task on the other hand.