wp-includes/class-wp-locale.php:381Checks if current locale is RTL.
bool public function is_rtl() { return 'rtl' === $this->text_direction; }Introduced in 3.0.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-locale.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.