Rockstar is officially decoupling Red Dead Online from Red Dead Redemption 2. That means you'll be able to buy the online multiplayer mode as a standalone product on both PC and consoles--it's not going free-to-play.
This new standalone version of Red Dead Online launches on December 1 and will cost $20. However, for the first few months of availability--from launch until February 15, 2021--it will be 75% off, selling for just $5. As an online game, it requires a subscription to PlayStation Plus or Xbox Live Gold if you're playing on PS4/PS5 or Xbox One/Series X|S, respectively.