Question about Orange box

It should be on there. But if not ? You can wait for the xbox x series or the xbox series S (the more affordable one )

And, if it were not backwards compatible, you might have to wait until the release of Xbox Series X, as they have said they have stopped working on bringing Backwards Compatible titles to Xbox One until the Series X launch to get Backwards Compatibility
up and running on the Series X. Form there, I imagine they will move back to bringing titles to both Series X and Xbox One for a few more years.

valve delisted the game several years ago and the only way to get it is to purchase a physical copy and use it on a system with a disc drive

These gamepack is Backwards compatible already

Here is the list of BC games: https://www.xbox.com/en-US/xbox-one/backward-compatibility/available-games