My iOS Deployment Target is 14.0 as I want to support both 14.* and 15.0 as well as under my Targets in developement Info. Thought that, in combination with conditional code via @avialable/#available, should be enough to support both, right?

I don't get it. Please clarify what you meant exactly with "build against iOS 14 SDKs" as I'm sure I'm doing it...