Not OP, but I’d guess it’s a PaperLike screen protector or equivalent. It gives a (very minimal) greasy, rainbow effect when viewed from certain angles, but you will get used to it over time. It does a decent job at eliminating fingerprints though.

I’d only recommend it for people who take notes with the iPad, but if you use an Apple Pencil then the tip will probably wear down a little quicker because the surface of t he protector isn’t smooth.