s&box is now open source under MIT license, you can get it on GitHub and build the engine however you want.
Obviously this isn't the Source 2 code, that's up to Valve to open source if they want. For us Source 2 is providing lower level systems, all our high level systems are C# like the entire editor, networking, scene system, UI, and way more..
What this means is you can view, modify, copy any of our code to help improve s&box with pull requests, or maintain your own fork for your standalone games, or even just take the code for your own engine.
It might seem odd from a business perspective to make an engine and give it away for free with no royalties and to give all the code away under open source. But we're a bunch of nerds that love what we're creating, we want everyone to use it in whatever way they want, we want to provide opportunities.
Open source is great for the game dev ecosystem, engines like Godot are awesome, we should have more of it because everyone wins.
It's a heavily modified and upgraded version of source 2, but yeah.