
Keys is a newly released single player Source mod, made mostly by Ryan Trawick, that is freely available to anyone with a Steam account.
... Which is made possible by Valve's generous licensing of their Source SDK 2013 Base. This is kind of a big shift in policy for Valve. Historically, mods have been locked to their parent platforms so that they could drive-up sales of triple-A retail (e.g. people buying Arma to play the original Day Z, or Warcraft 3 to play the original DOTA), but something here has changed. Perhaps Valve has decided they have enough money, or perhaps they realized Steam is already a powerful platform to lock-in people anyway. So now, Source 1 is kind of transitioning into more of a middleware platform like Unity or Unreal, though most people outside of the TF2 / CS:GO communities have generally moved on already.
What are Source mods in a "post-mod" age, where they're not even modding a retail game anymore, and they're freely distributed and shared? Can we even still call these things "mods", or have they transcended that type of framing?