It's a bit ambiguous. The
Apple App Store Review Guidelines don't go into this; the primary concern is preventing low-effort apps, abuse of the app store system, or inappropriate subject matter. Those guidelines might be said to incorporate Apple's
Human Interface Guidelines, because they tell you to "check whether your app follows guidance in other documentation," including the Human Interface Guidelines. Those guidelines simply say that "content typically fills the entire screen." It's also hard to imagine that those guidelines can be requirements because they're very high-level descriptions of design principles, e.g., "use metaphors." I doubt their reviewers really evaluate an app's adherence to best design principles in that manner, although they could certainly reject an app on that basis if they wanted to. The most direct guidance I've found is under the specific Human Interface Guidelines for
adaptivity and layout, which tell you "Don't attempt to hide a device's rounded corners, sensor housing, or indicator for accessing the Home screen by placing black bars at the top and bottom of the screen."
I've updated my interface so there are no black bars and to use the extra vertical space, although my game simply has no use for some of the extra side space on squat (4:3) ratios. I wonder if
@Bayesian has any basis for his or her "absolutely unacceptable" comment, although who knows, the internet hivemind says that Apple reviewers can be a bit unpredictable from one day to the next. Googling only finds one rejection for black bars, and that report is from 2014. It's unfortunate that the GMS community has little involvement by professional mobile developers. I think pricing the export initially at $400 pushed out most indies. Maybe we'll see an uptick now that the price is "only" $200 and GMS1 people are now forced to upgrade if they want to continue maintaining their games on the Play Store.