NOTE: if you rename the older version to Xcode-11.5.app you can run both side-by-side (same as running beta versions). There will be issues if you’ve upgraded a project to the latest Xcode format (e.g. after you get the warning to update project settings and choose to fix/OK them). Also, if you use command-line you can xcode-select to switch which version is used when running xcodebuild directly in a terminal. Anyway, just note it’s not as smooth as upgrading everything to latest.
NOTE 2: You also may find issues with debugging directly on device if your iPhone/iPad has the latest version, then I think older Xcode versions may not be able to debug on them? You can do a number of indirect methods to install and test builds from early Xcode on device, the easiest is either using an older test device that either cannot update beyond iOS 9.3 or using TestFlight since any device will run an app that has a Deployment Target <= device iOS version.
Just to be clear, it is very annoying how Apple treats Xcode releases, granted it is somewhat consistent with their actions on not supporting older OS versions.
For example, to very recent Xcode version, 12.5, is now tied to requiring Big Sur
And they fully changed how a lot of the IDE itself works, and made lots of keyboard shortcuts and other workflows no longer work as they did.
I am begrudgingly on Xcode 12.4 recently, but I think this thread is giving me the idea to go try downloading Xcode 11.x.
My point is that my advice may solve or at least help you with these issues, but unfortunately Apple doesn’t make it easy to work with older Xcode and latest iOS device versions, among other stupid things (this is one area I miss from Microsoft’s “backword compatibility at all costs” modus operandi.