A lot of porn from the 70's and early 80's is lost or obscure. Same goes for a lot of the Japanese porn, hentai, and extreme horror from the 80's and 90's VHS era. The infamous "Go For A Punch" guro anime is a well-known example and apparently, it's very recently been confirmed to exist and we even have a release date (Winter 1986) making it one of the early hentai titles of the OVA Boom but we still haven't found the footage itself.
One hardcore gore fetish movie from Japan in the early 90's was presumed to be damn near lost with only three surviving VHS copies, and it eventually was transferred to DVD.
There's actually a hardcore gay porn film from the 70's about a horny gay guy having a crush on Jesus and the local priest. It was called Him and was one of the OG "lost media" memes and holy grails online alongside The Day The Clown Cried. A lot of people were convinced it was a hoax given how ridiculous the premise was and because the book that first brought it to the internet's attention mentioned that one movie mentioned in the book was fake.
Turns out the fake movie was Dog Of Norway (and the screenshot was a photo of the author's dog) while several print advertisements and reviews for Him were later found in old archives and the existence has been confirmed. So far, not a single surviving print has been found.
If anything, porn is one of the genres most likely to fall victim to becoming lost media since it's a genre that's niche and works are largely considered disposable
