Expecting something is not a promise, just an attitude they want to have towards it at the moment. I think you are also confusing who is even in a position to p

Expecting something is not a promise, just an attitude they want to have towards it at the moment. I think you are also confusing who is even in a position to promise what. Kernel is not an OS people can use, but only something an OS (distro) itself can use and, given the license, in any way it wants. And so kernel cannot promise or force ABI compatibility or anything really on behalf of any OS that uses it. It's up to the OS, but OSes modify kernels, backport things, build with various ABIs and so on. Look for example at the mess around x32 ABI, some distros had it, some didn't, some had it and dropped it, some had and promised ABI compatibility for some time, but Linus wants to drop it from the kernel (don't know if he actually did it), so they are in a pickle. Read RedHat's application compatibility guide if you want an example on what the best a linux distro can promise wrt ABI compatibility.