I assume you mean the decline in the use of the term AJAX as it was now just the standard and you don’t need to use that to describe your site or tool as being capable of being highly interactive and dynamic vs just static.

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.