https://wccftech.com/apple-stops-signing-ios-13-7-downgrade-from-ios-14-no-longer-possible/

Apple released iOS 14 and iPadOS 14 last week and it brought a boatload of new additions to the table. The update is huge considering the number of features it brings to the table. Now, a week after the release, Apple has stopped signing iOS 13.7. What this means is that if iOS 14 did not entice you for some reason, you cannot downgrade to the previous iOS 13 build anymore.

iOS 13.7 was Apple's final version of iOS 13. Up until now, users had the option available to downgrade to the previous version. Now, Apple has pulled the plug to downgrade from iOS 14. Take note that the move will not hold any major importance for average users.