Free (libre) software sort of faded into existence, simply because the notion to copyright software faded into existence, and free software sort of relies on people to consider that software can be copyrighted in the first place. "A-2 System" https://en.wikipedia.org/wiki/A-0_System is roughly the first open-source software, although I'm not sure an explicit license text even existed.

The second question is what will enter public domain first, business-owned software or individually-written software. If the latter, which open-source developer has died the earliest?