The SDK has been published on github : GitHub - ValveSoftware/source-sdk-2013: The 2013 edition of the Source SDK
Wiki : Source SDK Base 2013 - Valve Developer Community
I don’t know if software rendering would be enough on Haiku, but just want to share this information 
I just had a look at it. The build system seems to use linux-specific containers.
Also, it uses the valve project creator (vpc) which I’m currently trying to
port.
Even if the games themselves build though, you still need the assets of the
games and they probably won’t work with the normal multiplayer, as that is
steam dependant
I’m confused. This has been published for over 10 years. Why are you posting it now?
iirc valve added the tf2 server and client code to the repository