RDR2 owners can play this game online for free without PS plus subscription until 27 May 2019.
Per the article -
To celebrate the occasion, Rockstar Games is making Red Dead Online available to all PS4 players, including those who aren't subscribed to PlayStation Plus, as part of a special bonus period. From now through May 27, anyone who owns Red Dead Redemption 2 on PS4 can dive into its online multiplayer mode. With all the new additions and improvements, there's no better time to see what Red Dead Online is all about.
SPoiler: it's about microtransactions.