There are upskirt videos on youtube as well. I remember people trading them on AOL chatrooms, random popular forums, and Usenet before that.
The only way you can stop it is by censoring the Internet, thus human thought. That's going to prove unpopular no matter where you fall politically, but especially bodes unwell here.
I don't think there was a dedicated one then (at least I never took time to look for them when I was a teen, more focused on whatever piqued my interest at that point) but you could find people requesting and filling requests for them throughout alt.binaries.pictures.erotica and associated subgroups. My teens were mid-late 90s.
I just googled and see there is an a.b.p.e.upskirts one now, but I don't know when that was created. Could've been there all the while.
The only way you can stop it is by censoring the Internet, thus human thought. That's going to prove unpopular no matter where you fall politically, but especially bodes unwell here.