While Red Dead Redemption 2 features some familiar places for you to explore, you can't revisit Red Dead Redemption 1's Mexico section. At least, you're not supposed to.
Now, some Red Dead fans have found a way back down south after all...
There may be spoilers ahead.
If you've gotten far enough through Red Dead Redemption 2's story then you'll already know you can go back and visit New Austin, the majority of the map area featured in the first Red Dead Redemption game. Mexico is there across the river, but there's no obvious way to cross.
You can ride your horse across to the Mexican shoreline, although an invisible wall stops you from getting any further. Topical.
Players such as YouTuber ZacCoxTV have now discovered a crack in this invisible wall - and a way to get into the Mexican countryside proper. From here you can explore the whole of the region, and while placeholder trees and grass are present, there are few buildings to speak of. The exception is El Presido, the fort fans will probably remember.
Both the hidden route to Mexico and El Presido are shown off in the video below.
It reminds me of players glitching into bits of the map in GTA5, such as its starting area, and then wondering what they may be used for (if anything) further down the line. The fact Mexico exists as a landmass suggests Rockstar may be keeping its options open for Red Dead Online - or it simply could just be there to act as background scenery should you ever go peering through your binoculars across the border.
Don't stray too far, however. Doing so will eventually lead you past the point where the map actually exists - and you'll go tumbling into the floor.