In brief: Rockstar's latest open-world titles, GTA V and Red Dead Redemption 2, feature expansive multiplayer modes that have long since grown into their own projects, largely isolated from their single player campaigns. With RDR2's online component, Rockstar will be doubling down on this concept by detaching Red Dead Online from the core game entirely -- you're now able to buy it separately, if you so choose.
Existing Red Dead Redemption 2 customers will not be affected: they will still own Red Dead Online and all of their progress will remain in tact. However, new customers will be able to snag RDO across a wide variety of platforms, including PC, PlayStation 4, and Xbox One, starting December 1. The game will also be playable on next-gen consoles through their respective backward compatibility features.
Red Dead Online's price starts at $5, but that's only an "introductory" offer. Come February 15, 2021, you'll have to pay $20 to access the standalone game. Naturally, console players will need an active PS Plus or Xbox Live Gold membership to play RDO due to its online-only nature.
Sadly, splitting up Red Dead Redemption 2's main story from its online component won't result in a file size reduction -- it'll still be 123GB for downloads. We assume that's because the single player campaign's content is still in RDO's files, you just have to unlock it via a separate purchase (we're unsure how much that will be, though).
All in all, Rockstar's latest decision does make business sense. There's undoubtedly a sizable portion of RDR2's playerbase that only cares about the online component, and never bothered with the main story at all -- the same applies to GTA 5 and GTA Online.
By letting those players buy into RDO for a cheap introductory price (even $20 will pale in comparison to the full game's $60 price tag), Rockstar is creating more opportunities to hook customers and generate revenue from microtransaction purchases.
And for players, nothing should really be changing here. As far as we can tell, you can still buy the complete RDR2 experience (including RDO) for full price, if you wish, so this sounds like a win-win situation.