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I don't know offhand, but I can tell you Red Hat kept kernel 2.6 all the way to RHEL 6 because they didn't want to break backwards compatibility, but some issues with Docker got them to update it for RHEL 7 and I think there was something else container-wise that caused another update for RHEL 8. Wikipedia says those versions were 3.10 & 4.18. RHEL 9 updated the kernel again but with IBM buying them and changing lots of their policies I'm not sure if it was something compelling or just fit in better with IBM's targets.