Pretty much the title, I don’t know anything about custom Android. Does the phone’s proprietary firmware not work with newer kernels?
Lineage/DivestOS’s builds for Pixels 2-4 are still on 4.x which are EOL, even if the version of Android is still supported.
GrapheneOS keeps to newer kernels than stock Pixel Android (I think it uses the cutting edge kernels and drivers). You can find their kernel updates in their release change log here: Releases | GrapheneOS
Cutting edge 
You can read THIS, it kind of answers your question.