We now tunnel IPv6 as well as IPv4. This means you can get connected to the IPv6 internet through Mullvad, even without IPv6 support from your internet service provider.
It is not yet enabled by default as it does not work on some Windows computers. To use it, get the latest client program. Go to Settings and enable Tunnel IPv6. Try to reconnect. If it works: great, you have IPv6; now you can forget about it. If it fails to connect: disable Tunnel IPv6 again; everything will continue to work exactly as before.
Previously the Mullvad client program just blocked IPv6 on the computer to avoid leaks. Now it can route all IPv6 traffic through the VPN server. Any problems caused by the block will thus go away.
Those not using the Mullvad client program can just add the directive "tun-ipv6" to their OpenVPN configuration file.