It’s a standalone multiplayer client and you can grab it for cheap until February 15th, 2021. After that it will be $19.99.
I guess Rockstar milks their fanbase best. If you somehow haven’t played Red Dead Redemption 2 and don’t really care about the single player experience you can grab the standalone multiplayer game now.
It’s also available on Epic Games, Playstation and Xbox services. Steam to follow shortly I guess.