Today I tried to join one curve object to another as a shape key, but the function is greyed out in the menu. Is "Join as Shapes" not supported for curve objects?
Both objects have the same number of verts/points. Actually, one was made by duplicating the other, so they have the same curve resolution and everything.
Currently Blender's manual does not mention a lack of support for curve objects:
Join as Shapes (Transfer Mix)
Transfer the current resulting shape from a different object.
Select the object to copy, then the object to copy into. Use this action and a new shape key will be added to the active object with the current mix of the first object.
Most of the functionality of Shape Keys holds true for both meshes and curves. So I'm surprised by this. I also tested this on other curve objects, and the result is the same. Also, I verified that Join as Shapes is still working as expected on mesh objects.