> Markdown was based on the "syntax" already being used informally in emails and on IRC.

News to me :-/

> So the author did do some searching to define the syntax.

I recall using tin/rtin in 1995, and people used the org mode syntax for italics, underline and bold (not that it made any difference). Same with plain-text email (I used elm, then pine, then mutt). Same with IRC clients - convention was the org mode syntax, not the markdown we have today.

The very first time I saw '**' for bold was in setext, circa 2004. People weren't actually using setext though; they were using *some text*, _some text_ and /some text/.

Here is a post from January 2001 documenting what the existing conventions were: https://everything2.com/title/conventions+for+plain+text

Here is the jargon file (maintained in the 90s by Guy Steele and ESR) that documented the the markup/typography conventions of the time: https://www.catb.org/jargon/html/writing-style.html

In short, no, I don't believe that the authors did any research. I think what happened is that they saw something like setext, though "great idea, lets run with that!", and did so.