This button is only available in the Shape Tool Toolbar. It's not within the Button Gallery.

I would suggest having this button available on the Straight Line Tool and the Freehand Brush Tool.

I'd like to see it in the Button Gallery so I could copy it and have access to it at any time. Bear in mind the reason that I would want this is if I was drawing with a pressure profiles and I need to reverse where the thick and thin of the line is.

To have to stop drawing, change tools just to access one button, and then change back to the tool you were using is inefficient and unnatural to a sketching type of workflow.