Some good news for fans of crazy sporty games like Riders Republic, as Ubisoft actually worked to get it working on Steam Deck / Linux.
In an announcement post on Steam they said: "As some of you may have noticed, some work has been going on behind the scenes to get Riders Republic running on Steam Deck. And we're happy to announce that you can now play the game on this platform!".
The game uses BattlEye, as it's a big multiplayer playground, and since BattlEye does work on Linux they've now gotten it enabled. You just need to ensure you have the BattlEye Runtime installed in your Steam Library. They also said they will continue to work on improving the "flow and playability of Riders Republic, but we're happy that you can now enjoy the game on the go".
It's still labelled as Unsupported by Valve for Steam Deck but that's an old rating, so Valve will need to re-check it.
You can buy it on Steam.