wp-includes/class-wp-query.php:4278Determines whether the query is for an existing category archive page.
$categoryint|string|int[]|string[]optional''bool public function is_category( $category = '' ) { if ( ! $this->is_category ) { return false; } if ( empty( $category ) ) { return true; } $cat_obj = $this->get_queried_object(); if ( ! $cat_obj ) { return false; } $category = array_map( 'strval', (array) $category ); if ( in_array( (string) $cat_obj->term_id, $category, true ) ) { return true; } elseif ( in_array( $cat_obj->name, $category, true ) ) { return true; } elseif ( in_array( $cat_obj->slug, $category, true ) ) { return true; } return false; }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.