I believe that Air new Zealand was the first
I don't know who was first but can recall, as an aero eng student in the late 60s/early 70s, having Wal Stack as an Industry lecturer.
At the time Wal was the boss performance man at QF and that operator had been using flex techiques since 707 days. Wal (being an ex-mil pilot, as I recall) had a mixed engineer/pilot view of life and kept a 1000ft ASD pad for the crews' mums and kids.
Lovely fellow and the most wonderfully dry humour.