We where told that Old Halas was gone atleast as far back as last Fan Faire when they first talked about New Halas. I think even V'halen said something about that way back when he was still here.
As for this being "PROOF" that the next expainsion is Velinous, while I know you really, really, really, want that to be the next addtion, but... um... I seriously doupt it.
First, it is WAY to early* for the devs to start dropping clues like this.
Second, {looks at Fens near Kern's Tower} just because something is made of Ice and Velium does not mean it has anything to do with Velinous.
Third, Halas was a Northern City on Antonica, so even IF it was a clue about the next expainsion* it would more likly point to adding another one of the mission Shadderlands Islands to the map, and not a content located at the opposit pole.
Fourth, it is almost certain that Velinous would come with a level cap increase, and as the pattern has been sence KoS alternating Cap increases with horizontal expainsion, that would make the next expainsion one without a cap increase.
Fith, lorewise it makes the most sence for us to take on Kelifrim on Vellinos, and sence he just know showed back up on the stage, I doupt we will be taking him on directly in the next expainsion pack, first we need to find out why he took the powers of the swords, and what his other plans are.
::EDIT::to make the format easer to read and to add the astriked info I forgot to add.
*remember Halas was INTENDED to be released at the same time as SF, so if the lore in Halas had to do with ANY expainsion it is far more likly that it has to do with the current one, then some future one.