Can't find the petrified oak slab anymore

It was intentionally removed from the creative inventory in the snapshots (it was explicitly mentioned).

https://www.minecraft.net/nl-nl/article/minecraft-snapshot-22w42a

Petrified Oak Slab has been removed from the Creative Inventory

It can still be accessed through commands

Please readd it to the creative inventory and add petrified birch slabs etc. and stair, fence, door and trapdoor variants and pressure plate and butron. 

The only reason the block exists is alpha and beta world compatibility, as there didn't use to be burnable slabs of any kind. They cannot be converted to normal slabs as that'll cause worlds to burn down.

The slab is the only one of its kind, and will stay that way.