Note Rhino 7 has more requirements for running on Windows Server. Please see Running Rhino 7 on Windows Server for more details.
Nothing prevents you from running Rhino 6 on Windows terminal services but you'll find that performance is terrible.
Terminal services works great for applications that are primarily text-based, or that update small parts of the screen infrequently. Rhino is not such an application.
Rhino is a CPU and graphics-intensive application. Most Rhino users want to run their screen resolution at 1280×1024 or higher, in true color.
For good 3D feedback and display, users expect 15 frame-per-second full screen view regeneration. This is equal to watching full-screen movies on a terminal server. The network bandwidth requirements for such an application are huge, and the end user will very likely experience terrible graphics performance.
Also, there are several operations in Rhino that are very CPU intensive (for example, Boolean operations and creating render meshes for shaded display). These CPU intensive operations can spike CPU use to 100% for several seconds. In a serious modeling session, this will happen regularly. You will likely see terminal server performance degradation for other users because of this.
We recommend that Rhino run on standalone computers with decent video accelerator cards.