Through a reliable source I just heard that tomorrow (December 19th) will be the last day that American Express will offer gift cards through cash back portals.
Will this be the end for good? Probably not. This happened before, as recently as September 2nd. See: “Amex gift cards withdrawn from portals”. Then, one month later, Amex gift cards were back. See “Amex gift cards return to cash back portals.” I expect the same will happen here.
There is no word yet on mile-based portals, but I expect Amex gift cards will be withdrawn from those portals as well. Currently, the best offer I know of is for 2X Virgin America miles. The Virgin America Elevate portal seems to think the deal will last until December 21 (Sunday). My guess (and it is just a guess) is that it will disappear early (maybe end of day Saturday).