I honestly do not think source code will be all that useful. Make it so redistribution, decompilation, reverse-engineering and reimplementation is legal after sales stop and that covers it.
You might need old binaries to build it, but shove those in a VM and you should be good to go. If they used Debian, then they could even publish the exact snapshot.debian.org date to download the binaries from, and which binaries.
Perhaps they had proprietary dependencies they couldn't get the code released for, but then you could port the source code to open equivalents.