Click-mapping came earlier, and there may have been a few places mouse-movement and cross-page-load session tracking some sessions, but I don’t think it was a “just turn it on and leave it on” thing for even most large sites. And a lot of early heat maps came from user studies, which is the right way to do that.

[edit] also, that just happened to be the first time I’d seen a single session represented that way, rather than aggregates. Again, it wasn’t some brand-new thing then, it’d been around long enough to have multiple companies offering it as a service, not just an internal tool at a couple giants.