Adobe Slabs were introduced with Patch R36 on November 15th 2016.
The crafting recipe for Adobe Wall is unlocked by creating Adobe Bricks from Mud in a Forge, crafting (or finding) an Obsidian Mining Cell (after crafting/finding Stone Mining Cells), and collecting caramel-colored Canyonstone from high up in Canyon-biomes which requires a Stone Mining Cell or better.
8 Adobe Walls each can then be crafted with 2 (blocks of) Adobe Bricks, 4 blocks of any kind of Canyonstone (like Ruddy Canyonstone, Dark Canyonstone or the rather rare caramel-colored Canyonstone), plus 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 Adobe Slabs is one of the requirements necessary to unlock the crafting recipe for Adobe Stairs.
Adobe Slabs are also one of the requireded ingredients to craft Adobe Stairs.