Update:
https://developer.apple.com/forums/thread/118599
The same thread was here and not specific to SwiftUI but this warning can be safely ignored according to Apple if it's a system class like "CKBrowserSwitcherViewController" . However, if the class that produces is a custom class, then it's a legit warning to you.
egrimo
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Since Parallels announced 16.0 with Big Sur support, you might check out that version and see if it suits your need.