I stand corrected, so crossparty is more complicated than I thought, though still not impossible either.OMG i agree with leeh on something �� �� ��.
I will just leave these posts from the horse's mouth to show what leeh is trying to put across -
This was from June 2015 -
This was from May 2016 -
Look at the source if you want more context for the questions asked.
Some games such as Rocket League, fighters, racing games, I guess these are very unlikely to get crossparty features, though they can still benefit alot from crossplay matchmaking.
MMOs probably are the type of games that would benefit the most from crossparty, and usually these already have their own party and friends systems anyway (especially if they started as standalone/non-steam PC games).
EDIT: How does Street Fighter V handle it? Does it have some sort of friend system? Does it allow for Steam players to challenge PS4 players and the other way around? Or is the crossplay just matchmaking like Rocket League?
EDIT2: So SF5 does allow for crossplatform challenges, it has an ID system separate from the PS4 and Steam IDs, users can search for other players with the SF5 ID, add them to their favorites list and invite them for non-ranked matches or something.