I can't believe that Xcode requires Catalina starting with 11.4. Are you guys at Apple living under a rock? To enumerate the problems Catalina has would take hours. I can't imagine any technical reasons for forcing Catalina on Xcode, this appears to be pure arbitrariness.
Just FYI: I'm not a Facebook engineer, I don't have 10 development machines at my disposal, I have exactly 1 iMac where I write my Apps on and where I do all the other daily stuff with. And given the current state of Catalina, it would be very unwise to upgrade that iMac to Catalina. Consequently I'm stuck at iOS 13.2. Maybe someone at Apple can explain why 13.5 requires Xcode running on Catalina.
I know what will be next: if anyone from Apple bothers to respond, the response will look like this: file a radr. Why do they do this? So that the problem can be moved out of the public eye and silently swept under the rug. While this might satifiy some managers, it will not solve the underlying problem: Catalina is not ready for production.
To cut off regular customers from a stable OS and force them to use a public Catalina beta is one thing, but forcing developers is IMHO a scandal.
