Dirt Slopes were introduced with update R36 on November 15th 2016.
2 of these Slopes can be made in a Processor from one block of Dirt each. No crafting recipe has to be unlocked for that.
Dirt is a very common surface block that can be found right under the top surface layer of many Biomes in all Creativerse Worlds, and also on the Fossil layer underground, accessible through shallow Caves.
Different from cubic blocks of ordinary Dirt, Dirt Slopes cannot be corrupted by Corrupt Bombs.
Dirt Slopes will not turn into Grass Slopes when placed next to blocks of green Grass.
Since update R43 ("Welcome Update") in May 2017, 1 Dirt Slope can now be processed further into either 1 Dirt Slope Inner Corner or 2 Dirt Slope Outer Corners. This does not require any crafting recipes.