in_category()

function in_category( $category, $post = null ) {
	if ( empty( $category ) ) {
		return false;
	}

	return has_category( $category, $post );
}

<?php
/**
 * Tests if any of a post's assigned categories are descendants of target categories
 *
 * @param int|array $categories The target categories. Integer ID or array of integer IDs
 * @param int|object $_post The post. Omit to test the current post in the Loop or main query
 * @return bool True if at least 1 of the post's categories is a descendant of any of the target categories
 * @see get_term_by() You can get a category by name or slug, then pass ID to this function
 * @uses get_term_children() Passes $cats
 * @uses in_category() Passes $_post (can be empty)
 * @link originally at https://codex.gechiui.org/Function_Reference/in_category#Testing_if_a_post_is_in_a_descendant_category
 */

if ( ! function_exists( 'gcdocs_post_is_in_a_subcategory' ) ) {
function gcdocs_post_is_in_a_subcategory( $categories, $_post = null ) {
    foreach ( (array) $categories as $category ) {
        // get_term_children() only accepts integer ID
        $subcats = get_term_children( (int) $category, 'category' );
        if ( $subcats && in_category( $subcats, $_post ) )
            return true;
    }
    return false;
  }
}
?>