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  • Luis Ferro thinks this is interesting:

That’s because Apple does not encourage the user to hold the phone upside down because if the phone rings while it is in that orientation, the user would have to twist it through a full half turn to answer it.

From

Cover of Beginning iPhone Development with Swift 3: Exploring the iOS SDK, Third Edition

Note

The upsidedown orientation is hardware turned off in iPhones. Even if active, it will not be selectable.