Even then usage of AJAX declined rather slowly as it was so established, and indeed even now it's still used by many websites!
Before the invention of the xmlhttprequest there was so little you could do with JS most dynamic content was some version of shifty tricks with iframes or img tags or anything that could trigger the browser to make a server request to a url that you could generate dynamically.
Fetch was the formalization of the xmlhttprequest (hence the use of xhr as the name of the request type ). Jquery wrapped it really nicely and essentially popularized (they may have invented async js leveraging callbacks and the like), the creation of promises was basically the formalization and standardization of this.
So AJAX itself is in fact used almost in the entire totality of the web, the term has become irrelevant given the absolute domination of the technology.