There's no requirement for always online for single player, IIRC you have to make one connection to Steam/EOS servers initially and then you can go offline/online all you'd like. I've taken a live game on the Steam deck offline in the car/airplanes and it's worked just fine. Just select Play Offline when you get to that point.
If you mean in SP the game always is listening online, you're right there, there's no "single player" so to speak, you're hosting and joining a server when you do that, but like Gamida said there, just choose Not Listed.