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.
