RethinkDB was built by an open source startup. They couldn't monetize it at scale, but the team did a great job of open sourcing and documenting everything when they shut down. This included previously proprietary extensions offered under an enterprise license. The community picked it up an ran with it.

More like software ER rather than software necromancy. That being said, there are whole industries of specialists who are brought in to resurrect dead software projects that are still used in production. For example, rewriting an existing COBOL project in C++ with matching byte for byte output (since that's what the factory hardware expected).

A program is free software if the program's users have the four essential freedoms:

The freedom to run the program as you wish, for any purpose (freedom 0).

The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.

The freedom to redistribute copies so you can help others (freedom 2).

The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.