Can I delete the Platforms under Xcode.app/Contents/Developper/?

Yes you can delete platforms from this folder

but make sure that you remove a platform and the simulator, otherwise Xcode will refuse to restart after

Example for Ios watch : remove WatchOS.platform and also WatchSimulator.platform

Don Mag advice is good.

I would not remove anything from the /Applications/Xcode.app folder as this could cause you issues running Xcode.

There is a folder in ~/Library/Developer though that stores downloaded version of the DeviceSupport for any version of iOS/tvOS/watchOS etc that you have been developing for. Every time an OS is updated another download appears in these folders and older version are not removed automatically.

Any of these can be removed as they will download again automatically the next time you build to an OS version. Though bear in mind the extra wait time needed on the first build of an uncache version.

I use an App from the App Store called 'DevCleaner' which allows me to select any DeviceSupport files, Archives (I tend to keep these) and Derived data. It's useful as it shows how much space each folder is taking up.

I select everything but the latest versions of the OSes I am developing for.

Hope that helps.