We had iOS devices updated unexpectedly to iOS 13, even though we had "Delay OS update availability" to 90 days.

Thanks to a deep investigation by Eric, we realized that the "Delay OS update availability" option applies only to the user updates. And that setting gets ignored when an "OS Auto Update Policy" is set.

To prevent others from making the same mistake we made, I suggest you change the label description of "Delay OS update availability" on the device restriction profile, to make it explicit that this will only apply to user updates. How about something like "Delay OS manual update availability"? The "OS manual update" will contrast with "OS auto update" in "OS auto update policy".

One problem with this: there is also the user settings of "Automatic Updates" in the settings app. This is also called "Automatic", but this on abides by the "Delay OS update availability" setting, while the "OS Auto Update Policy" does not... This is all still quite confusing...

Another place to clarify this would be a warning message on the "OS Auto Update Policy" page, stating that this policy will ignore the "Delay OS update availability" setting.