Today I received word on a Feedback, and confirmed in my own app, that this has been resolved in Xcode 13.3 (13E113).
galenohanlon
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Removing the extension I mentioned above does indeed work around the issue. That's not a solution for me, since I actually do need that functionality. Again, the code that's causing the non-public symbols to be added to my build is this: UIFont.withSmallCaps. I've yet to find anything suggesting that the used API has been disallowed.
My app is also being auto-rejected for referencing those same two symbols, coded as "ITMS-90338: Non-public API usage". The only related references in my code that I can find (or in my few dependencies) is the following UIFont extension, which to the best of my knowledge is referencing the latest, correct public symbols: UIFont.withSmallCaps.
I've tried Xcode 13.2.1, 13.3 beta 2 and 13.3 beta 3. I've also tried with and without the new build mode (defaults write com.apple.dt.XCBuild EnableSwiftBuildSystemIntegration 1).