So, this is frustrating, but Steam does surface this stuff in the Steam Deck Compatibility info section. I wish they would make this more plainly visible so that I can avoid buying games that do stupid stuff, but perhaps it's in their best interest to bury it like they do. If you go to RDR2's Steam Deck info, it says it only requires an internet connection during first-time setup. So, it can be played offline. You just need to run it online the first time.
And some of them are strange picks regardless. Yes, you can play Elden Ring offline, and it's great in single player, but it's also got a great multiplayer mode that it simultaneously A) does not allow you to play via LAN, and B) simulates in some places via NPCs for the inevitable day that the servers aren't running. Hitman 3 locks all of the unlocks and replay value behind an arbitrary online requirement. Red Dead Redemption 2's main appeal is likely the story mode that you can play offline, but it has a substantial live service component.
EDIT: Also, I'm not a Monster Hunter fan, but I would imagine most of them prefer the game in online multiplayer, and there is no offline multiplayer option.
Has anyone played Tales of Arise? I keep opening its store page but haven't pulled the trigger. I saw in the reviews that the DLC was polarizing but is the base game worth it?