Found it: https://www.flyertalk.com/forum/34896812-post4.html

I think there's the kind that you plug in directly, but I like this kind with the short cable/plug, which makes it a bit more flexible depending on the seat. I've been using one ever since I switched to the BT Bose headphones a few years back and now the Airpods. I hate having wires so this works pretty well for me. It's super-light and I just always keep it in my small flight case (with eye mask, ear plugs, pen, lip balm, and Airpods).