Rainbow Six Siege is getting crossplay, kind of. Ubisoft went into detail about the game's upcoming crossplay support during its Ubisoft Forward 2021 stream. Curiously, while PS4/5 and Xbox players will be able to play with each other, PC players will be excluded from that pool. PC players will be able to match with Amazon Luna and Google Stadia players, but I can't imagine the cloud versions represent many players.
Ubi didn't go into why this will be the case, but it almost surely has to do with the skill divide between PC and console players. Balancing mouse and keyboard versus controllers has been a consistent challenge as more competitive games have introduced crossplay, but an ultra-precise FPS like Siege gets particularly problematic when you start mixing input methods.
Aiming with a controller in Siege is way harder than with a mouse. It's not a fair fight, and that's not a fact that's changing anytime soon. I don't see how Ubi could've worked around this problem in a way that keeps Siege fair and competitive, so it makes sense to sidestep the issue altogether by keeping the communities separate.
The decision is a stark contrast to how Blizzard is handling Overwatch crossplay. PC and console players will be able to match together in groups, but competition will remain locked to your platform.
To catch the rest of E3, check out our full E3 2021 schedule.