Yes it is and it is terrible. The lack of upstreaming also makes things like applying the PREEMPT_RT patches much more difficult than it should be. You might be

Yes it is and it is terrible. The lack of upstreaming also makes things like applying the PREEMPT_RT patches much more difficult than it should be. You might be stuck at a version of the kernel where PREEMPT_RT was never released, not to mention you will be missing the PREEMPT_RT changes in the custom code.