Author here! Would love to hear examples of particularly good or bad tech debt tradeoffs people have seen over the years :)
FamiTracker (a music editor) is in a local optimum of tech debt. It's written in MFC and DirectSound, mostly works, but is near-impossible to port to Linux or extract the engine core into a playback-only library not dependent on the GUI. 0CC-FamiTracker performed aggressive refactoring with insufficient testing, resulting in more bugs than older versions of the program. It's worse for end users, but a better starting point if anyone decides to finish separating the playback core from the GUI (and fixing bugs along the way).