Just talked to the customer support rep at US Digital. She said the message comes up because the stock on the modules is limited at this time. She assured me that there are no plans for the S4 or E4P encoders to be discontinued. They are working to resolve the inventory issue, but plan to continue to supply them to FIRST teams.
I get the feeling that they might not be able to support an order of the magnitude necessary for the module to be incorporated into a consumer product with volumes of tens of thousands, but have no trouble supporting our small quantity orders.