Redfall is both single-player and multiplayer, whether you want to slay vampires with your mates or go at it like a brooding lone wolf. The downside, however, is that it won't let you play by yourself if you're offline.
This comes courtesy of Bethesda itself in a Redfall FAQ post. The question posed was, "Will playing Redfall require an online connection for single player as well as co-op?" Bethesda confirmed, "A persistent online connection is required for single player and co-op."
What this means is that you can't play Redfall offline, so if you're internet goes out or you're away from home, i.e. trying it out on a Steam Deck while traveling, you won't be able to access the game.
This is fairly common with newer games as most developers have opted to use DRM in a bid to stop piracy and copyright infringement. You have to be online so that the game can read whether you have its license or not - most DRM games then require an internet connection even if you're playing solo, but this has often drawn backlash which we saw with Diablo 3 and, more recently, Hitman 3.
However, while you need to be online to play single-player, you won't have to subscribe to Xbox Gold. This is only required for co-op, meaning that a stable internet connection is enough. That being said, the FAQ post also revealed that you will need a Bethesda.net account to play - this isn't paid, but it's an extra step to get into the action.
Redfall is set to launch on May 2, 2023, but according to Hi-Fi Rush leaker Horns, it, as well as Starfield, are in rough shape at the moment. Both allegedly need more development time, with Microsoft reportedly rushing both. We won't have long to wait to see how true these claims are.