I have been able to put my CabinZero backpack 44L under the seat on Intereuropean BA and LH, but always in J. Most recently KLM cityhopper AMS to DUB in June. It also got on with me in April in Y on GA, and QF to and from Uluru. It is also never full and i wear it down the ramp.
Classic Backpack - 44L Original Grey | CABINZERO