I've done a fresh install of Big Sur, installed Xcode 12.2, downloaded iOS 11 and 12 simulators through Xcode->Preferences->Components but can use only the ones for iOS12. Whenever I try to add a new device through Xcode->Window->Devices & Simulators only iOS12 appears in the dropdown for OS version. Selecting Downloading more simulator runtimes... option brings me back to the Components window where there are both iOS 11 and 12 checked.

Running following command in terminal shows that iOS11 simulator runtime is unavailable on Big Sur: % xcrun simctl list runtimes == Runtimes == iOS 11.0 (11.0.1 - 15A8401) - com.apple.CoreSimulator.SimRuntime.iOS-11-0 (unavailable, The iOS 11.0 simulator runtime is not supported on hosts after macOS 10.15.99.) iOS 11.1 (11.1 - 15B87) - com.apple.CoreSimulator.SimRuntime.iOS-11-1 (unavailable, The iOS 11.1 simulator runtime is not supported on hosts after macOS 10.15.99.) iOS 12.0 (12.0 - 16A366) - com.apple.CoreSimulator.SimRuntime.iOS-12-0 iOS 14.2 (14.2 - 18B79) - com.apple.CoreSimulator.SimRuntime.iOS-14-2 tvOS 14.2 (14.2 - 18K54) - com.apple.CoreSimulator.SimRuntime.tvOS-14-2 watchOS 7.1 (7.1 - 18R579) - com.apple.CoreSimulator.SimRuntime.watchOS-7-1