https://wccftech.com/red-dead-redemption-2-mexico-map-models/

Red Dead Redemption 2 doesn't feature a Mexico region, not counting the very small area that can be explored through a glitch, but it seems like data related to a bigger map is found inside the game's files.

GTAForums member VideoTech UK revealed a few hours ago that, included in the game's files, there are a lot of Mexico map models, including models used for some locations that appeared in Red Dead Redemption like Chuparosa. LOD textures have also been discovered.