My en.php line 398 follows:
return preg_match("#" . $c . "{2}$#", $str, $matches) AND $matches[0][0] == $matches[0][1];
Can't explain it, but your formatting is different -- note the spaces that are missing in yours.
The deprecation was implemented in PHP 7.4; I'm running a version on 7.4.19 with no issues. . .