Comment by zinekeller

Here is the problem though: let's assume that the developer really wants to release the source code, but the problem is that they have used a third-party library where they cannot legally release because they simply don't have the rights to it. This complicates open-sourcing that software (this is already proven from time-to-time, for example the OpenJDK Java runtime needs to and has changed third-party components like the colour management system, font renderer and media framework). This can be worked around on actively-developed software or if the community wants to be involved on open-sourcing that software, but if it is being abandoned I don't see the incentive to replace these components.

Moreover, comments. From expletives to actual trade secrets, a company needs to audit the code to ensure that at least the trade secrets have been removed from the source code. For example, even though Microsoft does not release the Windows/NT code widely, there are checks to ensure that expletives and comments that might cause severe repercussions (i.e. political statements) are not inserted. However, this is definitely a lesser concern to individual or small-group developers.

(Plus tooling issues etc, but considering that the massively-popular and ported Doom exists, it might be solved by the community. However, software which no longer has a community will encounter these issues.)