Rainbow Six Siege's development team has come out and said that they have no intention of releasing a sequel but rather continuing to support the title for ten years.
Rainbow Six Siege has been a wonderful success story. The game landed and didn't immediately set the world on fire, but Ubisoft and the development team seemingly never gave up on it and supported the title to a very healthy place now. It's clear there was a belief in the game, that it could take off, and that dedication has paid off. Now it is one of the most played first person shooters out there.
It doesn't look like it is going anywhere soon either. Speaking at their esport event, 2018 Six Invitational (via VG247), the developers revealed there are no plans for a sequel. While that might seem disheartening, it is actually a good as the team intends to stick by Rainbow Six Siege for ten years. This would see the game put out 100 operators over it's lifetime, which would be a pretty big feat.
Of course, this is an ambitious roadmap and I wouldn't take as gospel. 10 years is a long time. Interest in the game might wain, Ubisoft might want a new burst of revenue down the line, or perhaps a potential new console generation may make porting an ingrained online shooter difficult. For now though, it's interesting to know what the current thinking behind the series is. Rainbow Six Siege most certainly has a future and for that, fans should be happy.