Limestone Slopes were introduced with Patch R36 on November 15th 2016.
2 of these Slopes can be made in a Processor from one block of Limestone Wall each. No crafting recipe has to be unlocked for that.
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.
Since R42 in May 2017 inner and outer corners blocks can be created for Limestone Slopes too by putting them into a Processor. No crafting recipes are required for this.