How do I still have Internet connectivity after removing the default gateway IP address from macOS network configuration?
You have only disabled IPv4 by deleting the IPv4 default gateway.
You have not disabled IPv6.
You are using IPv6 to get to IPv6-capable sites.
SuperUser.com, surprisingly, does not appear to be IPv6-enabled yet, which is why you can't get to it. Google's properties, including YouTube, are usually IPv6-enabled.