by Technopeasant » 28 Nov 2020, 19:07
Not to be a pedant (or at least to explain my original intention), but FreeDoom isn't a source port since it is just a WAD file. As a game it is mostly played via PRBoom+ or similar ports because last I checked it was still Boom-tied (though I heard they were trying to make it vanilla compatible in future). I included Blood because although it doesn't have a source code release, there are reverse engineered source ports based on the Build and Duke Nukem 3D source code (most notably BloodGDX and NBlood). This makes these semi-free software, although the Build license isn't properly free (non-commercial use is prohibited; the Duke and Shadow Warrior code are GPL 2) and of course all the Blood game assets and logic are still proprietary.
As for UT99, have you heard of the new OldUnreal 469 Patch? It does wonders for modern system compatibility.