You don’t need magic or even combat to be an RPG, you need the systems in the games to be dependent on stats/dice rolls. Disco Elysium, for example, is absolutely an RPG, and there’s no magic or combat in that game. One of the big giveaways that a game is an RPG is if there’s something you cannot do because your character’s abilities aren’t high enough. And not a combat move or something, but actual progression in some sense (dialog options, areas you can’t enter, bosses you can’t defeat, etc).
What makes Diablo an ARPG is that it’s an even mix of action and RPG. It’s unfair to call it an RPG because so much of the gameplay depends on player maneuvering, but it’s unfair to call it an action game because there’s so much depth to the skill tree. Other examples of ARPG are:
Ys - basically, Zelda with JRPG-style stats
Dark Souls - they have their own genre now, but I still think ARPG fits to a T
Hogwarts Legacy - lots of RPG elements, but gameplay is action first
I think Diablomight be different enough from core ARPGs to define its own subgenre: loot-based, ARPG dungeon crawler. ARPG is perhaps my favorite genre, but I honestly don’t like Diablo that much. My favorite game series is Ys, which I think strikes a perfect balance between JRPG elements and action; I find myself taking the “if I can’t beat the boss, I need to grind a bit” approach, but I can also just “git gud” if I really don’t want to grind out a couple levels (might double the length of the boss battle though). In a game like FF, you just can’t make up for being under-leveled after a certain point, whereas with action/action-adventure games, levels either don’t exist or don’t really impact progression (they may add cool abilities though).
So that’s why I think RDR isn’t an RPG. Even if it has abilities, they’re really not central to the game in the same way they are with other RPGs. You’re not going to lose a boss battle because you’re low on some stat, nor will you be barred from some content, you’ll just have to do more minigames to increase it. So it’s more of an immersive action-adventure, where you need to interact with the games systems to continue the adventure (eat, wear the right clothes, etc), and if you get it wrong, just sleep and continue. It’s similar to Zelda: Breath of the Wild, which is absolutely an action-adventure game, and it borrows some elements from the survival genre.