As far as I know, it started with this reddit post. It was a request for a “built with Emacs” badge. Those badges were the type of thing you used to see on Web sites. I’m pretty sure I had it on one of my old Web sites1 but I don’t see them as much now. In any event, MichalNemecek had seen one for Vim and wanted to put it on his site. The problem was that he’d switched to Emacs and was looking for something similar for Emacs users.
Since then, I’ve seen several mentions of this page from Lulu Cathrinus Grimalkin that has a collection of Emacs badges. It even has the famous—or is it infamous—kitchen sink Emacs badge. If you’re into this sort of thing and would like to add an Emacs badge to your site, Grimalkin’s page is a handy reference.
I’m guessing that the reason you don’t see these very much anymore is that blogs and sites are mostly no longer handmade but use something like WordPress, Hugo, or one of the other site generators. That and the fact blogs are no longer the exclusive domain of nerds. These days, the average blogger has probably never heard of Emacs or Vim.
In any event, if you want one of those badges, now you know were to find it.
Footnotes:
After I wrote that, I remembered that I had something similar (but for Org mode) on the placeholder for my “new” Web site. Of course, that was 12 years ago so my point stands.