Limestone Slabs were introduced with Patch R36 on November 15th 2016.
The crafting recipe for Limestone Wall is unlocked by mining Limestone from the Fossil layer with at least a Stone Mining Cell, obtaining Obsidian (bars) by putting Obsidian Ore into a Forge, and crafting Stacked Stone Walls from ordinary Stone (and after crafting Wood Walls).
8 Limestone Walls each can then be crafted with 4 (blocks of) Limestone, 1 Stone Rod made of Stone, Limestone or Bedrock in a Processor and 1x Melted Wax made of Beeswax in a Forge or found in Treasure Chests.
Obtaining Limestone Slabs is one of the requirements necessary to unlock the crafting recipe for Limestone Stairs.
Limestone Slabs are also one of the requireded ingredients to craft Limestone Stairs.