ASCII schematics were very common for a long time because it was about the only truly portable way to represent them and the are extremely (as in EXTREMELY) small in terms of file size, particularly if they are compressed and the simplest run-length encoding can achieve impressive compression because they are mostly space characters.
Today, something like a PNG file drawn in something like Paint is a very close analog -- virtually every OS installation has something that can read and write such pictures. You can also get very small filesizes, as well, but it requires a bit more effort. People blindly post images that are hundreds of kilobytes or even a couple megabytes and, on the rare occasions when I want to modify the image to point something out, I am almost always able to reduce it down to a few dozen kilobytes and sometimes only a handful of kilobytes -- and all with just Paint.
Today, something like a PNG file drawn in something like Paint is a very close analog -- virtually every OS installation has something that can read and write such pictures. You can also get very small filesizes, as well, but it requires a bit more effort. People blindly post images that are hundreds of kilobytes or even a couple megabytes and, on the rare occasions when I want to modify the image to point something out, I am almost always able to reduce it down to a few dozen kilobytes and sometimes only a handful of kilobytes -- and all with just Paint.