There's a semi-documentary called "Dawson City - Frozen Time" that I highly recommend.

Dawson City in the Yukon was the last stop for a lot of film reels during the Gold Rush. By the time they got there, the relatively fragile nitrate films were often a little tatty, and the subject films weren't commercial enough anymore to be worth transporting any further, so they just disposed of them. But it was just one spoke on the wheel, plenty of other cities would have been the dead end for film reels like that.

In the seventies, a trove of these old dumped films happened to be discovered where there had once been an indoor swimming pool and ice rink. They had used the old pit as a dump to infill it for the rink, and the local permafrost happened to preserve a bunch of the film.

Many of the films recovered, even in their incomplete state, simply do not exist anywhere else, or weren't even known to exist. As I understand it, a number are completely unidentified - that is, we have part of the film, but we have no idea who is in it or what it's called or who made it.

That's just one city, and just a fraction of the films that one city would have disposed of, and we only have them due to civil and environmental happenstance.

It's not a myth, that stuff was just considered disposable, and it was disposed of.