The Baldurs Gate 3 Steam page has been updated and it contains a bit of information that looks like it has been overlooked. Under the features of the game, it has been listed that Baldurs Gate 3 will have Cross-Platform Multiplayer. The only game currently which supports this is Farming Simulator.
This is an interesting development and with Stadia the only platform announced I would think that Cross-Platform would have to include Stadia. This is on the back of it being announced that Baldurs Gate 3 will make use of Stadia Only features including Crowd Choice. Baldurs Gate 3 looks like it is being built from the ground up with Stadia front and center so it will interesting to see how this all develops going forward.