A securityflaw has been found (and fixed) in the xpilotserver. As a result servers with versions below 4.5.1 will be filtered out of the meta. You can still use the client, there's no problem with that.
This is the reason why so many servers have disappeared from XPWho and the XPilot welcomescreen. The servers are probably still running like they always have, but they are just not visible to the internet anymore.