The software industry should have long ago worked out conventions for the use of abandonware, and there is a sh*tload of it out there that can still be useful, but there seems to be no one or no entity still in existence who can manage the intellectual property rights. And there are others, the IBM TTS software, that IBM has left wide open on their servers, but where no official permission has ever been granted for anyone to use it, and many queries about doing so have been made.
Copyright for software should be much shorter than for other media, and if any publisher goes completely out of existence via actual closing, not being bought out and merged into some other company, that software should automatically be "fair game" for anyone to use within 2 years, max.