Join Date: Jan 2005
Location: New York, NY
Programs: UA, AA, DL, Hertz, Avis, National, Hyatt, Hilton, SPG, Marriott
Posts: 10,069
United had a "test" cabin of slimline seats on a 757 pre-merger. They were moving in that direction for some time.
As for oldest seats in the fleet, many 737-800s have original Koito seats in F, which date to 1998 or so. Much more recline than any current domestic F seats but they are well past their expiration dates and will be removed on ships getting NEXT.