The 1.24.3 rebase was done by hiddenasbestos in December 2020. hiddenasbestos also did the 1.22.2 rebase as well as the prior versions rebases for libretro. The 1.24.3 rebase was just recently copied into the libretro branch from hiddenasbestos’s github repository in June 2021.
It already contains the libretro stuff and CHD support since hiddenasbestos has been updating the saturn core for years.
There have been about a hundred commits to the main libretro branch over the past 2 years since the core was last rebased to 1.22.2 that are not in the new 1.24.3 rebase, but most of these commits seem to be code cleanups. There was an update to the chd libraries to the current version, but since the 1.24.3 rebase has chd support, it might be simple just to port the updated libchdr code in.