wp-includes/class-wp-widget.php:321callable public function _get_form_callback() { return array( $this, 'form_callback' ); }Introduced in 2.8.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-widget.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.