The era it was built?Hmmm, good question. The intent? The overall "downhilley-ness?
Follow up question:
What is the difference between an 80s/90s mountain bike and a gravel bike?
One theory I saw was that modern mountain bikes are too good. So that most trails people actually ride kinda boring. A gravel bike that has a lot of similarities to a mountain bike from 20-30 years ago makes them exciting again.