The user experience in RfK 21 is very similar to what was available in RfK 20, but there are some changes that are necessary to transition from 20 to 21.
The Reyes architecture and RSL support have been removed from RenderMan in version 21. RSL removal results in new C++ plugins for Light, Displacement Displacement, and Imager Shaders, replaced Sample and Display Filter plugins.
Lights are now C++ plugins. The lights that ship with RenderMan are the same lights used by Pixar Animation Studio. The PRMan GafferThree LightPackage (Add -> PRMan Light) has been replaced with individual light packages for each new light shader. Mesh Lights also have their own GafferThree light package. Mesh lights in RenderMan 21 must use the PxrMeshLight shader, in contrast to being able to use any light shader in RenderMan 20.
The two Light Filters that were shipped with RenderMan 20, PxrBlocker and PxrGobo, have been deprecated in favor of Pixar Animation Studio's Light Filters. Migrating from 20.x to 21.x there are a more powerful versions of the RenderMan 20 Light Filters. The PrmanLightFilterCreate macro has also been improved. It reads in args files for use with any light filters found on the $RMAN_RIXPLUGINPATH and can share light filters between lights. Because it is a macro, it will not receive these changes automatically. You will need to recreate the macro to use the updated version. In Katana 2.2 and newer, you can create light filters directly in GafferThree, so PrmanLightFilterCreate is not necessary.
PrmanGlobalSettings and PrmanObjectSettings