It's actually recommended in the included battery safety manual for the Mavic Air.
You are correct .. the only place you can find such a recommendation is in DJI documentation.
According to their Intelligent Flight Battery Safety Guidelines ...
Travel Notice
1. Before carrying the batteries on an airline flight, they must first be discharged to a battery level lower than 30%
Extensive searching doesn't show any aviation safety authority or airline in the world asking for this.
The only place I find any mention of it is in the IATA's guidance document for shipping lithium batteries as air cargo:
Documents and downloads complementing the current edition of the IATA DGR.
www.iata.org
The International Air Transport Association (IATA) is the international body that supports aviation with global standards for airline safety, security, efficiency and sustainability.
All airline lithium battery policies are based on the IATA standards.
And the IATA's guidelines for Passengers Traveling With Lithium Batteries:
Documents and downloads complementing the current edition of the IATA DGR.
www.iata.org
.... makes absolutely no mention of discharging batteries before traveling by plane.
The fact is that no airline recommends discharging before flight.
Neither do the FAA, CAA, or other air safety authorities.
It's only in forums like this that the myth gets spread.
If it was a thing, you would see the safety scanners at the airport checking every laptop, mobile phone, camera, iPad etc to make sure their lithium batteries were discharged (and you don't).
But you do see planes with power outlets to keep your devices charged on long flights!!