Great question — answering as the UphillTech team:
AFAIK. You only need a couple of adapters, Old school mono-mono to 3.5mm adapter. 1/4" inch to 3.5mm adapter if your headphones have 1/4" plug. Lightning to 3.5mm adapter if you have Apple Earpods with lightning plug. I've never been in a situation where I would need a volume limiter so I cannot comment on its usefulness. I'm not certain you need much more than that; I've not seen anything else in my 30+ years of travel (mostly between western Europe and America). Personally, I only carry a mono-mono adapter and a cheap pair of earphones (3.5mm) to connect to the plane entertainment system and a pair of Bluetooth in-ear headphones to use with my phone.
Full answer by Max: https://travel.stackexchange.com/a/125375 (CC BY-SA 4.0).
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