I don't follow the trends either, but it is fun to speculate and discuss what's out there now and what trends are still hot or seem to be on the upswing. I don't know that jewel tones and metallics ever went completely "out," but I have some old Rubber Stamper and Somerset Studio magazines from the late 90s and early 00s. Both the projects sent in by readers and the ads had a lot of deep blues, purples, and reds and metallics. Embossing was big in those days, not that it is out now. Wax seals or faux wax seals made with hot glue and pigment inks were sorto popular, too. There were also a lot of earth tones and southwestern stamps and card designs. I just think that we don't see those things as much now as we did 10 to 15 years ago.
Vintage and artsy stuff was popular, but the border punches, retro themes, stencils, spray inks, and other stuff that are hot now were either not being used at all or were not as prominent. Some of that has to do with new products coming along that are specifically made for stampers and scrapbookers. Certainly the colors that have been popular the last few years were not as prominent in the 90s' and early 00s. The lime, chartreuse, and apple greens of today were not too popular, IIRC. Gray was a neutral, but you didn't see it used with orange and yellow all that much like you do now. Gray was used with other neutrals such as black, brown, beige, or maybe a southwestern red.
The stamping books from those days also had a lot of similar stuff. I have an old book by Dee Gruenig, and the colors she used were shockingly bright by today's standards. Now that I look at the book, I can't imagine why I bought it, LOL.
I don't think that anything really goes away permanently. Trends come and go, and things rise and fall in popularity.