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.
This isn’t true at all. The general legal consensus is that foreign nationals are entitled to virtually the same rights as US citizens while on US soil.