I don't think I'm particularly old (only in my 40s) but I'm old enough to have made presentations with transparent sheets on an overhead projector. I don't quite remember how I prepared those sheets (I want to say a printing tracer?).
My HS teacher had elaborate combinations of transparencies that he would layer on top of each other. The upper layers containing hand-drawn marker annotations.
I’m in my early thirties and we sometimes used transparent sheets on overhead projector too when I was in school. Although in our case we made the sheets with the computer (Microsoft Word, Paint, etc) and they gave us transparent sheets that we put in the printer. I remember that the ink on the transparent sheets would smudge if we touched it, which sometimes accidentally happened when we handled our transparent sheets.
Even in high school I sometimes saw teachers use overhead projection with transparent sheets.
One of my math teachers in high school brought a special device with him one day. This one https://www.calculatorsource.com/ti-89titanium-vs.html (or some other model by TI). It’s an external screen that you connect to a TI graphing calculator and then put it on top of the glass plate of an overhead projector so that you can project what the calculator is showing for everyone to see :)
We had 35mm slides for customers but that was frequently an exercise in: "Here's our roadmap but this has changed and this isn't correct" because it took effort and time to get changes made once the slide was created.
It feels better than using Powerpoint.