When I worked for google as SRE for the web crawler, I low key advocated for de-indexing pinterest, but didn't get anywhere, (this is unsurprising, I was a nobody there.) I have no insight into why they don't de-index pinterest, but I know pinterest was on the radar as a thing of some sort. Meantime, I just add "-site:pinterest.*" to image queries... which I gather is probably more or less the sort of thing this addon does.