wp-includes/class-wp-rewrite.php:353Determines whether permalinks are being used.
bool public function using_permalinks() { return ! empty( $this->permalink_structure ); }Introduced in 1.5.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-rewrite.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.