The same problem is with timing charts: there just aren't suitable glyphs in the Unicode set for those, either. Tables and the simplest line charts/trees one can do, but that is about it.
One could add needed glyphs to the Unicode private use area to the fonts one uses, and that actually works well on the web, including forums like this one, where the modified font can be provided in OpenType/TrueType/WOFF2/WOFF format and render perfectly, including with hinting, in all browsers in all operating systems (exactly as well as native fonts). But it is not a reasonable solution for documentation in general, at all. It is much easier to use MarkDown or some other easy plain text format for documentation that supports e.g. SVG diagrams, make those schematics in Inkscape (yields very small, optimally crisp SVG files, also human-readable — I sometimes write SVG by hand, and many of my test cases output charts and diagrams in SVG), and a browser-based viewer that renders the HTML version real-time from the plain text formats, as used in GitHub and elsewhere.
ASCII art was a solution to a problem that doesn't exist anymore, but is somewhat interesting as an art form, as a variant of custom rasterization, where individual raster points themselves are images or glyphs. Some artists have used custom rasterization to highlight dichotomy/duality/dissonances in perception, with close view often a counterpoint to the far view.