It does not, sorry. The itch.io version comes with the game files and executables for 32- and 64-bit Windows and Linux. It has all the same features (except steam integration), will receive the same amount of support, and you can play multiplayer with Steam users.
In theory 127 players, but I would not recommend more than about 5. Players can join or leave at any point during a game.
The multiplayer is cooperative, but there is no ingame chat so it is best played with friends using a separate communications tool. (There are also not many players so do expect to have to organise a group to get started.)