if you’ve been following the various threads about GGUs over the past couple years you’re probably well aware that U availability is (1) extremely route-dependent, (2) extremely unpredictable in terms of how far out the flight date is, and (3) generally opened one seat at a time
searching yesterday for WAS-SEA on 16 Feb (Thursday before Presidents’ Day holiday weekend) I found availability from both BWI and IAD where both F cabins were ~13/16 open, but unsurprisingly none from DCA (4/12 and 12/12)