This community note is terribly worded, and is an example of relying on complicated industry jargon and "smart sounding" poor sentence construction that ends up confusing people.
Here's a plain English translation of when and why oxygen masks drop down in planes:
1. The air above 10,000 feet is too thin, give or take, for most people to breathe and stay conscious. The higher you go, the thinner the air.
2. Oxygen masks drop down when a plane's cabin loses pressure. Cabins are pressurized to the equivalent of 8,000 feet or so. This is how you can breathe without a mask while flying.
3. If a plane loses cabin pressure, the masks come down. They only have enough for about 10-15 minutes of oxygen.
4. Meanwhile, the pilots do an emergency descent to get the plane to 10,000 feet or below, so that you can breathe without the masks.
-J