"Pull requests" can also be done via a mailing list (it's literally a request to pull from some URL, see git-request-pull). It is not common on LKML, but I've seen it used that way. I think it's mostly maintainers that use it among themselves, everyone else just mails patches.

It's used by subsystem maintainers to request Linus pull their changes (it is literally the only mechanism Linus accepts from subsystem maintainers, outside of a few outliers like the -next tree). But yes, most regular developers don't use it -- it doesn't allow for inline patch-by-patch reviews like "git send-email" or "b4 send".