brycew, 
Off the cuff thought: I've seen a few places open sourcing code from the 80's and 90's (notably MS DOS 4.0 https://cloudblogs.microsoft.com/opensource/2024/04/25/open-sourcing-ms-dos-4-0/), now that the code's 30-40 years old, now that it's not viable to sell it anymore.
The max copyright lenght in the US was 42 years for a good chunk of our history.
There's no reason it should have changed. That code could have all been in the public domain by now
