Bedrock Slabs were introduced with Patch R36 on November 15th 2016.
The crafting recipe for Bedrock Wall is unlocked by mining Bedrock 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 (after crafting Wood Walls).
8 Bedrock Walls each can then be crafted with 4 (blocks of) Bedrock, 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 Bedrock Slabs is one of the requirements necessary to unlock the crafting recipe for Bedrock Stairs.
Bedrock Slabs are also one of the requireded ingredients to craft Bedrock Stairs.