Seems Bose stopped including the airplane adapter with their headphones a few rounds ago (probably the QC35 IIs).

They do have an original adapter, sold out on their website store, but available via Bestbuy: https://www.bestbuy.com/site/bose-qu...lack/6319328.p

(The Bose adapter is fine if you want the name-brand, though off-brand stereo-3.5mm—>airplane adapters likely will work fine too, they are not complex. I use a non-folding adapter with my non-Bose headphones which I got a while ago for ~$5-for-two, or one I believe from AA back when they gave out free earphones with an adapter included.)

The third prong is not necessary when using your own headphones. IIRC, it provides power for the ANC on some airlines’ non-battery headphones — and otherwise limits the airline headphones’ usage by people taking those home.