Red Dead Redemption 2 features an extremely large game world, one that becomes even larger once you finish the main storyline.
The fact that a part of the original Red Dead Redemption's map would be included in the new title was revealed before launch. However, the circumstances of this area being unlocked have only appeared after reviews were posted, and now it seems that there's more of the map in the game than previously thought.
In short, if you complete the main storyline of Red Dead Redemption 2, a large section of the map to the west of the area you've been playing in all this time opens up. That section happens to be all of Red Dead Redemption's map sans Mexico. You could see the other side of the San Luis River, but you couldn't go there. Or so we thought.
Fans became obsessed with finding a way into the Mexican region of the map. If it was at all possible, many were keen on discovering some kind of glitch, some kind of exploit that let them clip through the invisible wall - and find one they did.
As it turns out, the Mexican part of the map is entirely in Red Dead Redemption 2 as well, it's just blocked.
Many fans have documented the glitch on YouTube. Since it is entirely possible that multiple players discovered the crack in the invisible wall independently of one another, no single person can be credited with the discovery of the glitch.
Basically, you need to go to as far west in New Austin as you can, to the point right between the 'a' and 'd' in Seas of Coronado. You'll find a notable stone on the shore, and if you enter the water while on horseback just right of said stone, you can swim across to Mexico.
The map is rather bare, without NPCs and surprisingly enough even though most of Mexico is present, not all of it is. After a while, the map just... ends. It's one of those cases where it is possible to fall off the edge into the empty void below the ground texture.
Now, on the one hand, it would be great if this meant that DLC will open up the rest of Mexico - maybe even story DLC adding narrative missions in the region. However, realistically, this is just a skybox made more elaborate.
t's strange that Rockstar didn't just port the entire Red Dead Redemption map since the graphical touch up was likely procedural, mostly automated and tied to the engine which means that cutting off the bits that are missing probably took more effort than just leaving them there. However, since only the parts of the map which you see from the north half are there, the purpose of this snippet of Mexico might be purely aesthetic.
Nonetheless, this is an interesting development, and we hold out hope for some DLC opening it up properly.