In general, you see it more often for older websites or older server software, because we only really worked out around the year 2010 or so, that essential information for identifying a resource should be placed in the path.

Beforehand, it was largely something that webpage authors decided based on gut feeling...

Yup. I’m copying some Audible links now and the ampersand isn’t encrypted and the query string starts after the ? instead of the last slash, so there are different ways of doing it. We couldn’t guess at that, though! :)