The help card for the new polyextrude says that the Extrusion Mode parameter has four options:
- Primitive Normal
- Point Normal
- Translate Local
- Translate Global

However, when using the node I only get the first two. I am using Houdini V15.0.244

The documentation in that Houdini 15.0 build was not entirely up-to-date. That should already be updated in the documentation of the newer builds. You can see the updated documentation online as well:
http://www.sidefx.com/docs/houdini15.0/nodes/sop/polyextrude [sidefx.com]