is_protected_meta( string $meta_key, string|null $meta_type = '' )

function is_protected_meta( $meta_key, $meta_type = '' ) {
	$sanitized_key = preg_replace( "/[^\x20-\x7E\p{L}]/", '', $meta_key );
	$protected     = strlen( $sanitized_key ) > 0 && ( '_' === $sanitized_key[0] );

	/**
	 * Filters whether a meta key is protected.
	 *
	 * @since WP-3.2.0
	 *
	 * @param bool   $protected Whether the key is protected. Default false.
	 * @param string $meta_key  Meta key.
	 * @param string $meta_type Type of object metadata is for (e.g., comment, post, term, or user).
	 */
	return apply_filters( 'is_protected_meta', $protected, $meta_key, $meta_type );
}