I'm unable to find any word on when Apple will be requiring new submissions be made with Xcode 13. We've already moved over to it, but still need to know.
You should better check News and Updates.
(Link shown below every page of the developer site.)
App Store submissions now open for iOS 15 & iPadOS 15
Starting April 2022, all iOS and iPadOS apps submitted to the App Store must be built with Xcode 13 and the iOS 15 SDK.
Unless the detailed date is announced, you should better take it as the first day of April 2022.
