It's well known that id Software's Commander Keen tech was originally designed to accommodate a PC port for Super Mario Bros. 3, and in 2015, John Romero revealed footage of the proof of concept. Nintendo didn't go for it, but it was a breakthrough in terms of bringing smooth screen-scrolling to PC games.
A copy of that Mario demo has turned up in a submission to the Strong National Museum of Play, seemingly at random. The museum's games curator Andrew Borman tells Ars Technica that the disc was among a larger submission from an unnamed game developer. This developer didn't work on the demo, though received it "during their work."
"It wasn't something I expected to see in this donation, but it was extremely exciting, having seen the video Romero shared back in 2015," Borman said. The curator imaged the disc, booted the demo, and found it to match up with Romero's 2015 video. In addition to the footage we've seen, there's reportedly a "fairly flat" Level 1-4 in the demo.
It's good to hear the demo will be preserved, but according to the Ars report there aren't plans to exhibit it to the public. Researchers "and other parties" are welcome to submit requests to access it, though.
Romero's footage of the demo is below, and it's well worth reading Ars' report for more insight into the preservation plan.