-e: the material preview window has a bunch of primitives to choose from but no plane but you can set it in the shader itself. so it'd be nice to just have it out of the box

-repro:
--create new material
--select the material
--cycle through the primitives in the preview window at the bottom of the inspector
--notice there is no plane
--create new shader
--add Tag "PreviewType"="Plane"
--add shader to material
--select the material
--NOTICE there is now plane (but all the other primitives are no longer available)