Copyright terms last effectively forever, and certainly a lot lot longer than 20 years. https://en.wikipedia.org/wiki/Copyright_term

In a sane world it would have expired, but not here.

cf. The Mickey Mouse Protection Act https://en.m.wikipedia.org/wiki/Copyright_Term_Extension_Act...

Copyright tends to run from the death of the author for a period (in the US is 70 years after death). As for where it's assigned to an institution, it'll still have a long time to run - decades!

Copyright is either the death of the author plus a period, or a length of time for a non-human entity such as a university or limited liability company.