About 20 years ago I and two others designed an embedded kernel that switched tasks with setjmp and longjmp. It was exactly as you suspected—the only implementation specific part was creating a jmp_buf on task creation (which doesn’t require even assembly intrinsics). We were only targeting one architecture/abi at the time (arm) but it was nice to know that porting to a new architecture wouldn’t require a bunch of assembly rewriting (the only assembly was the interrupt trampoline—even our crt0 was C (well, there was at least one intrinsic asm instruction to set the stack pointer in crt0)).