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.
It’s not console exclusive, though