That's probably left over from the TeamSpeak 3 closed Beta program when you had to login to a "beta portal" in order to get your personal copy of the TeamSpeak 3 client back in the day.
By the way: 3rd level domains don't get "registered". 2nd level domains do.
I suppose they just didn't bother to remove the DNS records for that sub-domain when the beta ended or simply forgot about them.