Machines: Wired for War is a true classic 3D RTS from the late 90s, and it appears to now be open source under the GPL and up on GitHub. This was in the list of actually being one of the first proper 3D games of the genre, although not as well known as many other RTS games.
Back in 2019, one dedicated fan posted on their website about how they managed to grab a copy of the source, update it a bit and port it to modern platforms (Linux included). The actual rights to the game appear to sit with Nightdive Studios now, and they appear to have given the greenlight on open sourcing the code from that same fan which they've now listed on their own official GitHub.
Nice to see another bit of gaming history not being forgotten, open source is a truly wonderful thing for game preservation. Surprisingly, the original website for it remains online, which is like looking into a time machine itself.