There's a bit of a difference between an in-game text chat and a whole new social platform specifically for a game.Rocket League uses the platforms friends system for party play, partying with players from other platforms would need an in-game friend system, which is something that Rocket League devs would have to implement, I guess.
If MS isn't stopping in-game communication (in-game text chat), then I don't see why they would be stopping an in-game friend system.
It won't happen for these reasons IMO:
- Large development effort for teams to re-invent what's already present to them: XBL, PSN, Steam etc all have a platform which they hook into. Why re-do that?
- Creates segregation within platforms: Since the 360, all titles have hooked into XBL and only XBL for social actions. Moving away from that would segregate player bases, add complexity for the end user and would be up to the developer to maintain rather than the platform holder (Could go down etc).