TYPO3 Core

Instead of removing the handling of _GP(defVals), it needs to be deprecated in FormEngine. I think FormEngine is public API and there is no way to find out if the current version handles _GP(defVals) on its own or needs proper configuration from outside. If you set configuration from outside your code will throw an error if the new configuration is not supported (e.g. with 9.1).

As there is no method that can be used to determine if configuration is needed, I think the _GP usage needs to be deprecated "softly".