Have LEGO stickers changed materials over time?
Note. Speaking from my experience from year 1990 onward. During this time I can distinguish at least 3 periods of different experience with stickers.

From 1990 till sometime before 2000 (or early 2000's) there was period of superb stickers. Plastic was thick and glue was superb. I don't know what else LEGO used by then but most of them have survived my childhood abuse. Yes, they look rough, but they are there where I've placed them about 30 years ago. Both, clear and opaque.
Now second period begin somewhere early 2000's. Cannot tell exact period, speaking from experience of used sets I bought from that period. This is the worst time. Stickers applied onto sets during that moment not only cracking, but also peeling off badly. Plastic used for stickers seems to be thinner and glue is not as strong as it was before. Another odd thing is that different colors of the same stickers peel off differently. Some colors (like black) stick a little better (than white).
Last period is what I would call &quot;modern&quot; days. Must have started somewhere before 2013 when I have returned from dark age. Plastic for these stickers feels to be same thickness as previous period, but they seem to behave a little better. Perhaps not that much time has passed for them to deteriorate yet.
Looking at pre-1990 sets and minifigures some of them seem to have had paper based stickers (like minifigure torso prints). I have none of these, so cannot tell about their properties.