That’s obviously the people who are really using it, but I assumed you had to pay Apple to get it.
So either you made your own measurements or if you were doing Made For iPhone and they would give it to you as part of that program which surely you must have to pay for.
Commoditize Your Complement - Apple wants accessories to be cheap.
You might want to pay for the Made For iPhone certification for some things, but I doubt most cases have it.
I assume there's NDAs in place with some vendors pre-release, since my understanding is some accessory vendors get the needed info before new models are released, so they can spin up their lines and have accessories ready to go when the new model hits the market.
(but I could be wrong; NDAs w/China? no idea what the time between announced/released/shipped is typically; I last bought Apple product in 2009...)