Not true. This is something I had researched in the early days. If you can store a crypto hash of the document on a Blockchain, you can upload said document anywhere you want for people to DL. The hashes will always match, and you can use the timestamp of the Blockchain entry as reference to veracity. This could be done via an open source project and should the maintainers fall off the map, some one can always fork.

I remember seeing this listed as something like “proof of existence”.

In Tokyo they had a Stranger Things themed restaurant for a while. Pop up culture is pretty big in Japan so it worked out great I suppose. I don’t expect it to work in other places nearly as well. Especially if it’s a permanent structure.