It appears the offer of a Family Gold Pack Subscription, which cost $99 per year and allowed four family members to have Xbox Live Gold access, has been discontinued.
According to an entry on the Xbox Support Forums from one of their official support techs last night, a Mister Ohnaka, it was confirmed that the offer is not being offered any more. He did indicate, as you can see above in the screen capture of the response, that the Family Center would remain available to current Family Gold subscribers presumably until their subscription expires.
The $99 plan was very flexible and actually would allow you to assign anyone to one of the four slots in the family plan and they did not necessarily have to be actual family members.
I guess it is possible that the plan was being abused in this way and that might have prompted Microsoft to cancel the service.
No matter the reason it now means a family of four will have to be pay somewhere between $160 and $240, depending on the deals they may find, per year to get the same level of services the Family Gold Plan offered.
Thanks to @MasterDevwi for the pointer to this on Facebook.