The purpose of Minecraft From Scratch (MFS) is to remove features from the game in order to match the features of an older version of the game (currently 1.8) in order to provide a barebones base for other modpacks to add onto with the classical era of Minecraft in mind.
I don't know why, how, or when, but Superflat worlds do not appear to work correctly due to the changes to the worldgen.
Due to the drastic changes surrounding villages in modern versions of the game villages have been removed entirely and will need to be reworked to match proper classic villages.
Probably not. 1.19.4 was selected because it was the latest stable version at the time MFS was concieved and the point of MFS is to not incorporate features of modern versions of the game save for enhancements and bug fixes.
Probably not. You are welcome to port it yourself, though.
Minecraft from Scratch still has a long way to go before it can be considered complete due to the issues but my hope is that with some work it can serve as a whole new way to mod the game from the ground up.