Mark the function with an attribute to allow the old behavior. See link for similar use case.
https://www.php.net/manual/en/class.allowdynamicproperties.p...
https://www.php.net/manual/en/class.allowdynamicproperties.p...
Thus if understanding this right that would require to update every function signature to new behavior rather than marking a few functions to get the old behavior and automatically get the new better behavior of every other function for free.