Yeah a lot of retro games on GOG were fixed up with patches and stuff like that (often by GOG themselves) and sometimes regardless of any fixes applied, there are version disparities between the two platforms where usually the Steam versions is a slightly older release of an old no longer updated game compared to the GOG version though I’ve seen it happen the other way around, too.
I’d also recommend BZFlag, Xonotic, OpenClonk, and OpenLieroX. A surprising number of people are still playing BZFlag. Xonotic has some of the most fun weapon mechanics I’ve seen in an arena shooter and pretty much all of them can be used for movement shenanigans in different ways. OpenLieroX has a wealth of fun mods to mess around with.
Would you count GZDoom and games that use its engine? Due to its license, devs of even commercial games made with it have to publish source code if they modify the engine and the very nature of the iwad files it uses means that all of iwad’s game code (I guess except compiled ACS scripts, which people don’t use as much anymore) is in plain text.
Also I kinda still enjoy Red Eclipse 2.0 even if I found the balance and gameplay of 1.6 better and you can kinda mess around with the weapon variables until it does feel the same. Used to enjoy doing that a lot with 1.6, even doing stuff like turning the shotgun into a chaingun.
Why do you recommend liquid war 5 over 6?
It’s likely for 'user friendliness’. Most people don’t even know what an archive is and that it should be extracted so a folder is much more intuitive and familiar to them.