macOS 10.14 has been EOL for more than 2 years now and basically every Mac released since 2012 is compatible with 10.15. Valve also didn’t actively flip a switch and disable functionality; they’re just no longer providing updates. I don’t think Valve shoulders any blame in this specific case - it’s unreasonable to expect any company to indefinitely support platforms that are effectively obsolete.
Microsoft also released their own package manager called Winget a few years ago. It mostly just wraps existing installers to allow for unattended installation, but it seems to work pretty well in my (limited) experience.