"xcodebuild -downloadPlatform iOS" command in terminal also fails. I was able to download 18.2 from https://developer.apple.com/download/all/?q=ios and install it by "xcodebuild -importPlatform ~/Downloads/iOS_18.2_Simulator_Runtime.dmg ". Although there is a note that "future simulator runtimes will no longer be available on this website" and 18.3.1 is already unavailable.