Developer API: Provide notifications (webhooks) of de-listed families?

The Developer API serves data on the families listed in the fonts.google.com directory; however, as we de-list older deprecated families (like Droid in #1223) then then Developer API also de-lists them.

We should consider providing notifications (webhooks) of de-listed families, or move them to a tail end root node section of the returned dictionary that the developer API returns.