@Luca1719 I'm afraid that doesn't work. Xcode (12.0 beta 4) always displays the svg with black paths as soon as I drag it into an .xcassets.

@vslavik: While this works, wouldn't it mean that the Mac App Store still shows the old icon, though?

This solution ceased to work with the latest beta. Does anyone know an alternative? I really don't want to display my Big Sur app icons in previous macOS versions. Changing the DockTile is suboptimal as it only works after the app has launched, of course.

What finally worked for me was installing the macOS 12 beta from inside the recovery mode: Download the Monterey installer app onto your internal SSD. Boot your M1 Mac into recovery mode by shutting it down first and pressing + holding the Touch ID key. Connect your external drive and format it using Disk Utility; choose APFS as file system. Disk Utility can also be used to unlock your Mac's internal SSD in case it's File Vault 2 encrypted. Still within the recovery mode, open Terminal, navigate to the Monterey installer location (probably ~/Downloads on your internal disk) Start the installation process by executing ./Install macOS 12 Beta.app/Contents/MacOS/InstallAssistant Note that the installation may take significantly longer, especially towards the end.