physical security is also critical (Score 1)

Assume you have figured out how to package your computer time capsule. Where will you store it for 50 years?

If it is stored in a school system attic, basement, storage area, etc., you can count on it that some administrator or custodian will decide to throw it out to make space for something else they want to store.

A city or university library that is not part of the school system may be more likely to keep your package for 50 years. Maybe not.

The same argument applies to every kind of site I have been able to think of. At the least, plant multiple capsules in different places.

I am not trying to throw cold water on the time capsule idea. It would be a good project to build a capsule or capsules and store them, even none of them survive.