Just change it to wp_is_valid_utf8 that will make the warning message go away.
same issue here
deprecated: Function seems_utf8 is deprecated since version 6.9.0! Use wp_is_valid_utf8() instead. in /var/docs/server.tst/public/wp-includes/functions.php on line 6173
\bbpress\bbpress.2.6.14\bbpress\includes\common\formatting.php line 810 !