wp-includes/class-wp-customize-manager.php:496Returns the Ajax wp_die() handler if it's a customized request.
callable public function wp_die_handler() { _deprecated_function( __METHOD__, '4.7.0' ); if ( $this->doing_ajax() || isset( $_POST['customized'] ) ) { return '_ajax_wp_die_handler'; } return '_default_wp_die_handler'; }Introduced in 3.4.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-customize-manager.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.