If I understood your proposal correctly, to get the new behavior add an explicit stop to the function, my proposal add attribute to keep old behavior.
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.
Oh okay, you would do it in reverse. I strongly agree with that. But that means the new feature is opt-out rather than opt-in, and it may break some old code. Maybe it should be done in two-steps (opt-in + deprecation, then opt-out).
Typically you emit E_DEPRECATED for a full major version, then in the next major version you throw an error, e.g if it would land in PHP 9.0 then E_DEPRECATED for non-compliant functions and in PHP 10.0 start throwing errors.
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.