Foreknown

@MergingApollo Spent time modding yesterday and today, all mods have been working as anticipated as well without any extra tweaking. So can confirm Rockstar Launcher works on Linux via Heroic Games Launcher (with some tinkering) on CachyOS and modding works as well.

Also, the mod I believe you are looking for is "Red Dead Offline" as mentioned previously here.

Yoooo Just did this myself last night as I was curious and missed playing RDR2 (used to play on PS4 a long time ago). I am also on Linux (CachyOS) and after a bit of tinkering I was able to get Rockstar Launcher running through Heroic Games Launcher. I am starting modding today, so have yet to give confirmation on how well that works specifically, but I can tell you that you CAN run it on Linux if you are up for a few steps of tinkering. I wouldn't buy it again through steam if you already purchased and put time into it! (Screenshot for proof)