I had never heard of Setext! I do know that, before Markdown became popular, lots of programmers had their own similar format going on - this was mine, from 1998:

Perl program https://sourceforge.net/p/rosegarden/code/HEAD/tree//branche...

Documentation https://sourceforge.net/p/rosegarden/code/HEAD/tree//branche...

I wonder now how much of this was picked up unknowingly from Setext-derived documents.