Another user has posted a link to a mod for 1.12 that seems to do cubic chunks. I wouldn't have thought networking would need to be altered for servers where all users have the appropriate mods installed on the client; world generation and entity AI would follow already-established rules about how loaded chunks relate to unloaded ones.
Basically any part of the code that reads/writes chunks in the world or just references some kind of chunk-related data-type would have to be altered