https://www.gamespot.com/articles/rainbow-six-siege-no-sequel-is-planned/1100-6456867/

Rainbow Six Siege might not have gotten off to the best start, but it's doing much better now in terms of player figures and general sentiment--and it's not going anywhere soon. Brand director Alexandre Remy said in an interview that no sequel is planned, at least not yet. Instead, you can expect Ubisoft to continue to support Siege through the games-as-a-service model.

"We are saying extremely loudly here: there is no sequel planned," Remy told Eurogamer. "We will be here for the next 10 years. So expect more Rainbow 6 in your life for quite some time."