I would recommend Monodraw: https://monodraw.helftone.com

Apparently the author used `vim`, but you can also do this in `emacs` with `artist-mode` and `ditaa`[1]

[1] https://www.youtube.com/watch?v=cIuX87Xo8Fc

He says it's vim's visual block mode + colour post-processed in GIMP

PlantUML can also generate diagrams and export as ASCII art.