AFAIK its Monodraw: https://monodraw.helftone.com/
(I've asked Brandur this in the past, so assuming he hasn't changed it this should still be what he uses)
(I've asked Brandur this in the past, so assuming he hasn't changed it this should still be what he uses)
My internal ikiwiki pages are filled with ASCII art diagrams. This tool looks awesome; I’ll have to try it out!