I wish Valve would take a cue from id and release the source code to Source and/or at least GoldSrc under an open license. Though I have a hard time imagining them opening up code to more lucrative games like Team Fortress Classic or Counter-Strike classic. Nice dream though...
Old-ish Source has a pretty generous license, no? https://github.com/ValveSoftware/source-sdk-2013/blob/master... GoldSrc is there somewhere as well. I’m definitely grateful for it. I’ve read some of it myself and watched a bunch of Youtube about it.
Why should Valve give up their leverage. They actively pick and choose who they license the engine to.
Source uses licenced middleware such as Havok so SDKs are likely all we're going to get officially. Source 2 might be a different story if HL3 releases one day since Valve replaced audio and physics with in-house technologies.