It’s the stuff we used to use to protect our exercise books at school. You can buy special laminating vinyl but this stuff is cheaper.
Acid free sticky tape (scotch tape in the US?) will also work for smaller stickers, just make sure the sort you use doesn’t yellow. In my experience, glossy photo paper scratches easily and has to have a layer over it to seal it, plastic is the easiest option (also remembering when I made over 200 trading cards on photo paper and had to design, cut and laminate them all by hand lol).
I did test the brand of vinyl paper I used with water and it did hold up. I’ve only had it on my bike for a few months, but so far it doesn’t seem to have yellowed or faded. I probably should have cut out the sticky back plastic to be bigger than the sticker though.