You do know Valve could have not use Chrome and just spent money to build it right. Don’t defend Valve for being cheap.
The technology used by Valve is Irrelevant. The operating system losing support is not even supported by apple. The users of that version of MacOs are at risk because they use a closed source unmantained operating system.
As I said Apple is not concerned with kind of old software. They expect everyone to move up with them, developers and users, or get left behind.
Portal is a game released THE SAME YEAR the iPhone was. In classic hit PC game time that’s “nothing”, you expect to be able to run it, but in Apple’s timeline is ancient history. Take a look into how many iPhone games just won’t work anymore.