wp-includes/class-json.php:945Calculates length of string in bytes
$strinteger function strlen8( $str ) { _deprecated_function( __METHOD__, '5.3.0', 'The PHP native JSON extension' ); if ( $this->_mb_strlen ) { return mb_strlen( $str, "8bit" ); } return strlen( $str ); }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-json.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.