Starting with Rhino 7, McNeel introduced native SubD (Subdivision Surface) objects. This is the direct, built-in replacement for T-Splines and is highly compatible with NURBS workflows.
The T-Splines technology was absorbed by Autodesk and is now a core part of the Sculpt workspace in Fusion 360 .
Autodesk no longer develops or sells the T-Splines plug-in for Rhino. buy t splines for rhino
If you specifically need the T-Splines toolset, you can subscribe to Autodesk Fusion 360 , which allows for interoperability with Rhino.
Since you cannot purchase T-Splines, most Rhino users have moved to one of the following modern solutions: Starting with Rhino 7, McNeel introduced native SubD
Replacement for T-Splines - Rhino for Windows - McNeel Forum
A popular third-party subdivision modeling plug-in for Rhino, often cited by the community as a powerful alternative for organic modeling. Autodesk no longer develops or sells the T-Splines
If you already own a license, you may still be able to use it with Rhino 5 (64-bit) , as that was the last compatible version. Modern Alternatives for Rhino Users