Also, I'm pretty sure that this has been available for more than ten years, just not on GitHub.
It has been--that's where I started learning C++.
:)
Doesn't an SDK automatically includes at least some usable subset of the engine, if not the whole part?
No, just enough for you to plug your own code (gameplay, custom weapons, vehicles, etc). It's essentially a bunch of headers.
It's more than a bunch of headers, the Source SDK contains the code for
both `client.dll` and `server.dll`. They contain everything that is not
the graphic/audio/net engine.