There is no in-kernel API/ABI. The Kernel team is very vocal that the only interface that exists for the kernel is the userspace systemcall/filesystem interface

There is no in-kernel API/ABI. The Kernel team is very vocal that the only interface that exists for the kernel is the userspace systemcall/filesystem interface, and any code that wants to use anything beyond that really should live in the kernel tree. Things like the nvidia closed-source driver that hooks into the kernel itself are not using any API/ABI, they are basically just hooking into structures where the people maintaining those structures don't just not care about being compatible, but are actively hostile to people relying on their code remaining same across versions.