People used hidden iframes as a fallback for browsers that did not support XMLHttpRequest, but not before XMLHttpRequest existed. XMLHttpRequest shipped with IE 5.0, which also happened to be the first browser where "DHTML" support was good enough that building anything resembling a modern web application was actually practical.

We aren't talking about modern web applications and all that entails though, we're specifically talking about the ability to load and send content without reloading the page. That was in use in browsers older than Internet Explorer 5.

Netscape Navigator 4 had frustrating limitations once you had the data, but people were doing all sorts with it regardless. More was possible with Internet Explorer 4, but again, you didn't need XMLHTTPRequest. And if you just want to strip it back to the bare-bones of loading and saving data, a few people were even doing that as far back as Netscape Navigator 2 if I recall correctly – by dynamically loading new images with varying query string parameters.