In addition to this, Steam provides an option for developers to expose the older game versions to the players, which Valve themselves make an active use of. So if you have a specific old mod that’s not compatible with the new update, or don’t want to deal with the update in a middle of playthrough, you don’t have to upgrade
Maintaining the build artifacts and pipelines and testing backward compatibility for a long-lived project like HL2 must be pretty difficult, I would think? That’s a great example and counterpoint.