Xcode 11 Beta 5 - Loading a plug-in failed

Problem is gone on Catalina build 19A558d when installing Xcode 11 GM Seed (build 11A419c, released September 10, 2019)

I was able to get my SwiftUI project running on Catalina beta 4 with Xcode beta 5. I too got the warning mentioned by OP.

And as mentioned in comments, the only negative impact I've seen is being unable to use the preview canvas. Other than that, everything else in my project is running well. Just using the simulator more ;)