Mixpanel estimates iOS 15 adoption on 20% of compatible devices. Probably, adoption is lower due to new update process introduced by Apple in iOS 14.
Mixpanel: iOS 15 adoption
Reference: iOS 15 Suffering From Several Bugs as Adoption Rises to Nearly 20%
Recommendations
Developer:
Business as usual.QA engineer:
Business as usual.PM/DM:
Plan features accordingly.