wp-includes/rss.php:369 function is_rss () { if ( $this->feed_type == RSS ) { return $this->feed_version; } else { return false; } }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/rss.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.