This is inconsistent with sandstone, red sandstone, quartz and stone brick stairs and slabs which can be crafted with any variant of its respective material.
MC-135079
Purpur stairs and slabs cannot be crafted using purpur pillars
Resolved
Fixed
1
Confirmed
Minecraft 1.13, Minecraft 18w30b, Minecraft 18w31a
Minecraft 18w32a
Retrieved
This feels like an inconsistency with those other blocks rather than with purpur pillars.