Mayan Stairs Outer Corners were implemented during the May 2017 launch update, together with the other Roof and Slope Corner blocks.
Corners are created by putting Hidden Temple Stairs blocks into the processor.
Hidden Temple blocks are part of the Hidden Temple blueprint and block-set. You can buy either the Hidden Temple super bundle, which gives you the recipes, or the Hidden Temple pack, which gives you a sample of each block, but not the recipes.
The blocks can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.