In general, open-source all abandonware. Here's a few examples of non-game old software that would benefit the society if open-sourced:
Windows 9x and classic Mac OS. No one has sold these OSes for 20+ years and they turned out to be evolutionary dead ends. It would make sense to release their sources so people could learn from them and run them on new hardware more easily.
Flash Player. Adobe has stated in no uncertain terms that Flash is dead. Is there any good reason why the sources need to stay closed if they aren't going to make any further gains, financial or otherwise, from this product and its ecosystem? I mean the Flash Player plugin and standalone app specifically, not the Flash authoring software — that's still alive under a new name, Adobe Animate.
Trident, the Internet Explorer engine. Again, it's dead for good as far as Microsoft is concerned, so why not release its sources for people to learn and hack on?
Presto, the Opera engine. This was leaked and of course I hoarded it, but it would be nice to see an official release.
Winamp. The company behind it pivoted to some kind of streaming social donation thing. It does not seem interested in maintaining the old Winamp app.
Path, the social media thing for "close friends", their mobile apps. The apps were insanely cool, especially at the time of Path's peak around 2013, but the service was shut down a few years ago. Unless they're going to somehow resurrect the service, there's no good reason for the sources for the apps to remain closed. I'd love to take a look at their implementation of real-time photo filters.
Thinking of it, it makes more sense for non-game apps than for games to release sources. Old games do sometimes come out as "remasters" for modern platforms. These usually reuse the engine and sometimes some of the assets. Non-game software, on the other hand, when it's dead, it's usually dead for good.