Legacy is pre-bedrock and it’s NOT decompiled; some consider it to be the “last good” version of Minecraft that wasn’t Java - and perhaps could be used to do something interesting on non-jvm platforms.
Decompiled code is great, but having access to the “real” code gives you more insight into design decisions and directions.
Almost no coverage on HN or mainstream media though. Surprising, considering the popularity of this game.