Looking at the help docs for this material shader it says that there is a “use point alpha” attribute, however in the build I have (12.0.581) there isn't. Was this functionality removed on purpose?

I can't speak as to whether this was on purpose or not, looking at the Surface Model VOP inside the Clay shader SHOP which actually manages the Alpha and has this parameter in place but not hooked up as an input parameter, it will automatically default to pick up any point attribute called Alpha and uses this to multiply the Alpha channel. It's always on with the default Clay shader.

Given that the other Use Point Color parameter is available as a shader parameter, I am suspecting that this is an oversight and therefore a bug.

Please submit this bug to support at sidefx.com.

Submitted, and thanks