The closest to "forest porn" was when I was 8 or 9 and found a folder stuffed with porno magazines under my seat on the train. Of course, I was with my grandmother at the time so she made me leave it where it was. I was convinced I would find it again on the train on the way home and somehow sneakily bring it with me, but of course that never happened.
I guess that doesn't count because it wasn't at all a secluded area. I have never found randomly occurring porn in the wild.
I've honestly never encountered such a thing before, and thought it was another instance of the podcast making some sort of a funny joke.
The first time I ever heard the term was on the podcast. I asked some of my friends during lunch one day (we're all in our early-mid 20's) if they had ever experienced such a thing when they were younger, and all of them but one said "no". The one that did find it, however, lived in a very largely crowded neighborhood next to a large patch of trees, where he found it in a hollowed out tree. The rest of us lived in the countryside with not much population, so I can see how the 4 of us had never experienced it. He said it was about a box of 20ish magazines he said, all looking worn out.
So apparently it does exist?
Then again, with the digital age surrounding us, and the necessity of printed publishing diminishing, I doubt that this is a phenomenon that will last very long.