I can pick up some writing on physical media -- an Akkadian clay tablet -- and read it (if I have the knowledge) despite it being thousands of years old.

Things like laserdiscs, I can probably still buy equipment to read, but it's substantially different as I need the technology to read it.

Microfiche is quite good in this respect, you can easily read it even without the specific tech it was made for (using a magnifier, or projecting the image with a simple light source.

I wonder if you could make a crystal where, like a hologram, you can rotate the crystal a minute amount in order to project a different page (an idea I saw decades ago had a digital clock style projection from a crystal, used asa sundial -- pretty sure it was theoretical).

That way the information is relatively easy to discover, and with a simple light source you can get info out if it.

I keep hearing the fear of losing content because of obsolete file formats.

Then I think about Linear B, and I rest again.

https://en.wikipedia.org/wiki/Linear_B

There's no emulator that can run a Linear B parser. If a linear B dictionary ever existed, it was never mass produced. Linear B is older than the printing press, let alone the Internet. But now we do have those technologies, and "lots of copies makes stuff safe" is cheaper and easier than ever. I don't believe any mainstream digital format (i.e. popular enough to have a Wikipedia page) will be permanently lost unless there's a complete collapse of society, and then we'll have bigger problems to worry about.