Hook into `wp_enqueue_script` and defer all scripts using defer strategy from WordPress 6.3.

defer_all_scripts.php

<?php

/**
 * Recursively add the defer strategy to a script and all its dependencies.
 *
 * @param string $handle The script handle.
 * @return void
 */
function recursively_add_defer_strategy( $handle ) {
	wp_script_add_data( $handle, 'strategy', 'defer' );
	$script = wp_scripts()->registered[ $handle ];
	if ( ! empty( $script->deps ) ) {
		foreach ( $script->deps as $dep ) {
			recursively_add_defer_strategy( $dep );
		}
	}
}

/**
 * Hook into wp_enqueue_script and defer all scripts using defer strategy.
 */
function add_defer_to_all_scripts() {
	$scripts = wp_scripts()->registered;

	// Iterate through all enqueued scripts and add the script strategy
	foreach ( wp_scripts()->queue as $handle ) {
		recursively_add_defer_strategy( $handle );
	}

}
add_action( 'wp_enqueue_scripts', 'add_defer_to_all_scripts', PHP_INT_MAX );