You can see basically all that’s new in RenderMan 23 in the video above, plus Pixar has a great landing page with more information about the release (I am slightly obsessed with the way they’ve used the Duke Caboom asset to help showcase 23).
The big developments revolve around USD, interactive workflows and interaction with SideFX Houdini.
It’s also worth checking out the RenderMan 23 Documentation, which will also take you to the release notes.