Nice, this also includes the infamous half-life 2 leak from 2007 as well as some other ones that definitely weren't officially released. Or like re-volt, which is now owned by a company still actively trying to take down mirrors of the code that pop up, hence any unofficial port of it only releases in binary form. But then again a lot of the older leaked ones, nobody probably cares about too much. There's definitely…
Game companies are learning old IP can be modernized for fresh new profits so most likely a lot of old game copyrights will become retroactively enforced and sources hunted down and expunged as they're remastered and made exclusively available in whatever online shop or subscription service. https://www.polygon.com/2018/7/22/17600008/nintendo-roms-law... https://www.maxim.com/entertainment/best-video-games-remaste...
I'd even argue that you could be smart as the IP holder and release the code with a clause saying any modifications and extensions must be open source too and the IP holder had the right to use those and make money off of it. Then you can decide if you wanna take the risk of potentially working for some company for free when you take on porting that 90s racing game to the PS 4 in your spare time.
Taking down leaked source is obviously pretty simple due to DMCA, but as history has shown that just leads to groups working on it behind closed doors and only doing binary releases. Which might be technically illegal too but probably much more involved for the IP holder to prove, so they don't bother in most cases.