This repo is for Red++ v3, which is the latest playable version. The repo for v4 is not in a playable state at the moment, but dev will be resuming soon.
RedPlusPlus was made private since people kept trying to play it way before it's ready. (Its development is on pause for other projects like rgbs (map stitch.)
The repo's description contains https://github.com/TheFakeMateo/RedPlusPlus, which is now a broken link. Has it been moved?