Can friends on different platforms play together?
The FAQ states that cross-play is not available at 1.0, and the subsequent 1.1, 1.2 and 1.3 update records do not announce it. Friends split between PC, PS5 and Xbox Series X|S cannot rely on the official cooperative mode to join each other across those platforms.
Is crossplay planned?
The developers are investigating it, subject to technical and platform requirements. There is no confirmed implementation or release date to plan around. “Being investigated” is not the same as “coming in the next update.”
Crossplay is not cross-save
Two-player cooperation, cross-platform matchmaking and transferring saves are separate features. This platform answer does not establish Steam–GOG interoperability, cross-save or every possible same-platform combination. Check a specific platform combination before buying another copy to join a friend.