While reviewing the results of my script that repairs broken images across the network, I noticed there are many images where the alt text is "a busy cat". It might be an older version of the automatically generated "enter image description here", but I don't remember it, and it appears in posts as recent as 2019. Given its prevalence and the variety of authors, it's likely to have been generated by something, but what?
It is part of the inline help for the editor:
Images are exactly like links, but they have an exclamation point in front of them: ![a busy cat](https://cdn.sstatic.net/Sites/stackoverflow/Img/error-lolcat-problemz.jpg) ![two muppets] : https://i.imgur.com/I5DFV.jpg "tooltip" The word in square brackets is the alt text, which gets displayed if the browser can't show the image. Be sure to include meaningful alt text for screen-reading software.