Valve was a completely new company then. They weren’t going indie, but Sierra didn’t pay them for the remake of Half-Life. In the documentary they talk about financing it by creating Half-Life: Day One.
Almost all games are supported on Linux, with the exception of games with invasive anti-cheat, but those rootkits shouldn’t be installed on Windows either.
The problem is that it’s unenforcable. I bet that’s one of the reasons valve is rephrasing.