Even having access to the source code might not be enough: I’ve seen cases of weirdly forked repos with custom additions and half baked backports that relied on

Even having access to the source code might not be enough: I’ve seen cases of weirdly forked repos with custom additions and half baked backports that relied on hacked system libraries and weird mix of specific legacy packages in some symbiotic way that just one specific laptop of some long gone developer was able to compile it and virtual machine of this Windows XP was passed around.