Actually, POWER9 RHEL7 has Linux 4.x, where x depends on the minor release -- unfortunately not the latest on the system I use. I think aarch64 is similar, but I'd have to look for rpm to check. They need similar attention, of course.

Anyway, RHEL kernels have various features backported to the vanilla version on which it was originally based, not just security patches, which probably makes the job harder. It is a major effort.