may trigger the problem in apps which don't otherwise use the concurrency APIs
That's exactly what happened to me - Realm brought them in and Touchgram 1.3.1 failed as a result. As I don't install any analytics or crash tracking, it took an in-person encounter with someone running 14.8 to work out what was happening.
Lessons learned, even if Testflight is of limited use (iMessage app extensions are more complicated case), it's still worth doing a smoke test. Also, keep some devices around on older iOS - I had nothing left between 12.5 and 15.0. Took 10 tries to find an old iPhone on eBay that's not been updated from 14 to 15.