Why did iOS have an encrypted keel?

It was recently reported that the upcoming version of iOS has stopped storing the keel encrypted on the device storage. The piece suggests that this might be a mistake, because it weakens the operating system against reverse engineering or something. Apple later confirmed that it was no mistake, since the keel does not contain private user info, there's no need to encrypt it.

This doesn't make much sense to me. Apple's iOS Security Guide makes no mention of encrypting the keel. Is this even really a thing? And if there's no need, why did Apple previously encrypt the keel in the first place? What security benefit would an encrypted keel even confer, since iOS devices encrypt the whole filesystem anyway? Wouldn't the keel have to be decrypted to be executed? Do other operating systems encrypt their keels for some security benefit?

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