Not necessarily source code, but anything that would keep the game in a playable state after end of life. So any one of:
- a) patch to game the game playable in single-player mode locally
- b) binaries to self-host a server
would satisfy that requirement.
Note that the details of implementations of any law that would advance the cause are still to be discussed, which is the whole point of the petition.
More on what would be expected and some example use-cases are available in the FAQ of the Stop Killing Games official web-page: https://www.stopkillinggames.com/faq