I did just that same thing for a client. They were using React to make 1 AJAX request to Foursquare. The thousands of lines of code turned into 22 lines of vanilla Javascript. I was shocked.
https://hackernoon.com/how-it-feels-to-learn-javascript-in-2...
Did anyone get fired for that bit of programmer malpractice?
With some effort, the relevant jQuery methods that act as polyfills could be extracted.
However, a minified version of core jQuery is < 20Kb. A very low technical debt trade-off to support 99% of browsers.
Reasons to include inefficient code for compatibility reasons might include applications where maximum reach is valuable, or where the scale is so large that the "infinitesimal market share" percentage plays out into thousands of end users.