GameSpy never "became open-source." It shut down. OpenSpy was created as a clone, so that people would still be able to play their GameSpy games online.
If Epic Games Store shuts down, there won't be an "open-source clone." Your only "alternative" would be either another store, like Steam, or piracy.
Heroic will let you add other non-Epic games to your library. You also may want to take a look at Playnite, which also has a variety of themes available:
https://playnite.link/