As @PeterFotiadis said, Autodesk will no longer develop new versions or sell the T-Splines for Rhino 6. It still works on Rhino 5 though. Autodesk has integrated T-Splines in Autodesk Fusion 360.
There is a similar plugin named Clayoo.
And while the SubD Object is under WIP. by Rhino itself, you’ll have to wait until Rhino 7 is released.
https://discourse.mcneel.com/c/plug-ins/clayoo
https://www.tdmsolutions.com/clayoo/
https://discourse.mcneel.com/c/serengeti/sub-d