Hey, can the game be, not local, but steam multiplayer? (asking because steam multiplayer needs having steam installed,  and this is an external application) 

Hi! From what I understand from this, you mean using the Steam Multiplayer APIs right? From what I'm reading from  https://partner.steamgames.com/doc/features/multiplayer, it's intended for games on Steam that wants to use their infrastructure for online multiplayer instead of using the dev's own dedicated servers. Now this is a game jam, and I am pretty sure that releasing a game on Steam takes some time for them to review the game, and you also need to pay an upfront cost, and the game cannot be played on itch.io as well if I understood it right, only on Steam. From my point of view, it does not make sense to integrate Steam Multiplayer to a game made for a game jam on itch.io. 

However, we did say you can use whatever tools you want, but it is recommended that the game can be played or downloaded on itch.io so it can be accessible to everyone. Hope this helps, if I understood it wrong please tell me!