I guess I probably misspoke. I was thinking about https://android-developers.googleblog.com/2017/08/hardening-... ... but those certainly don't require userspace changes (probably). And even the case where you'd want the new kernel features (but can't upgrade due to driver ABI incompatibilities), they've backported to several old kernels that are in wide use in Android... though I note that my 2-year-old phone is on an older kernel than most of those features were backported to.