The official Rainbow Six Twitter has made an announcement that all the Rainbow Six Siege servers are being taken offline for emergency maintenance. Though the expected downtime is only 30 minutes, during that time no one on any version of Rainbow Six Siege will have access to multiplayer.
We are bringing all servers down for emergency maintenance.— Rainbow Six Siege (@Rainbow6Game) December 1, 2016
Expected downtime: 30 minutes.
Thank you for your patience.
Though Ubisoft hasn't commented on why the emergency maintenance is necessary, perhaps it has something to do with the upcoming Year 2 updates coming to Rainbow Six Siege. In any case, the game will be down for only a short time if Ubisoft's estimates are correct. If they're not, we'll be sure to inform you of any delays bringing them back online.