Yolu need to merge in changes to the lineage OS source and then compile it. Not a simple task. Just think, if someone out there isn't doing it already (ie LOS 14,1 for any device with latest sec patches), obviously it's not a simple thing to do.
Security patches are not just changes in the kernel; more so the ROM. Remember, its the ROM that records the security patch level/date, and any compatible kernel/device/vendor could be used against it. You could use a modified kernel from LOS 14.1 (ie manually "upstreamed" via minimal changes) against a LOS Q ROM, and the security patch level would be up to date, not at the kernel source date (ie LOS 14.1). Or upstreaming the LOS14.1 kernel to a Q Kernel (and making it backward compatible with 14.1) then building LOS 14.1, will not change your security patch level.
Even building a ROM isn't a walk in the park. Especially for a new device that has no support. Which is where I'm currently standing. It's pretty easy to compile a kernel and configure one if you have the sources. But updating it or the security patches, especially on a device that's so old (however I still use mine almost daily), isn't easy regardless of your professional background. However for a little comparison on kernel differences across devices, the xt907, built around 2012 or 13, has a newer and better kernel then the tablet I am currently writing on as we speak, Amazon Fire HD 10. And this tablet was built no less than a year-and-a-half ago. However I find it a bit more difficult to build for the xt907 then I do my tablet.
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