On June 3rd, a few days after Open Fortress and TF2Classic decided to come back online, Valve edited a documentation page that outlined the expectations and do-nots of distributing mods of Source Engine games. Among other changes was a big one under the question “How can I get source code access from Valve?”

“We have in the past granted access to the source code of some of our games to a small number of modding teams who could demonstrate a need that could not be met by the SDK. We may do this again in the future under exceptional circumstances but because of the associated support burden and risk of leaks our default answer is to develop based on the SDK.”

More context is required here: Valve had a pseudo-system in which interested modding teams with needs that exceeded the capabilities of the standard Source SDK could get in contact with the company and, after signing an NDA, acquire the source code for the Source Engine itself (and various games on that engine) along with a chunk of Valve’s internal asset repo. For non-commercial projects, this was entirely free! It enabled a lot of really cool work to be done. Now, it’s likely gone for the foreseeable future.