wp-includes/class-wp-query.php:4436Determines whether the query is for a feed.
$feedsstring|string[]optional''bool public function is_feed( $feeds = '' ) { if ( empty( $feeds ) || ! $this->is_feed ) { return (bool) $this->is_feed; } $qv = $this->get( 'feed' ); if ( 'feed' === $qv ) { $qv = get_default_feed(); } return in_array( $qv, (array) $feeds, true ); }Introduced in 3.1.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-query.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.