UPDATE: The rumor in question has been confirmed to be fake.
A new rumor has surfaced claiming that Rockstar Games is currently working on a remake of Red Dead Redemption that will release sometime in late 2020 or early 2021, which presumably suggests it's in development for both PS4 and PS5 and both Xbox One and Xbox Scarlett, and maybe PC as well. It also shares a few interesting tidbits about the remake, alongside revealing that Rockstar is also working on Red Dead Redemption 2 story DLC featuring aliens (more on that here).
The rumor alleges that the project is more of a remake than a remaster, and that it brings over the features from the new game. In other words, it makes it more in-line with Red Dead Redemption 2 gameplay and features wise. Meanwhile, the map is also being made bigger, though it won't include Red Dead Redemption 2's map.
The rumor also claims that Rob Wiethoff -- the voice of John Marston -- has come back to record new lines of dialogue to help connect the two stories, in other words, lines that reference Arthur and the events of the first game. It's unclear if other actors received similar call backs, but you'd assume if they brought back Wiethoff, they'd bring back others as well.
Apparently, the remake was supposed to release on the game's 10-year anniversary, which is next year, but development issues might mean it's pushed to 2021. Further, some of Red Dead Online team has been shifted to the project to ensure it releases on time. And this is perhaps the bit that sticks out and potentially discredits the rumor. There's certainly a lot of reasons to release a remake of the first game -- namely making a TON of money -- but to pull people from Red Dead Online to do so seems odd, because there's much more profit potential in Red Dead Online, but at the moment it's not being realized, mostly due to slow and lackluster support.
That all said, take this with a huge grain of salt. This is a rumor from an anonymous source who claims to have a friend that's an environment artist at Rockstar Games. It's all a bit shoddy, and, as you will know, rumors pertaining to Rockstar Games are infamously unreliable. However, I've seen plenty of shoddy rumors and leaks like this turn out to be true, so who knows.