Exclude out of stock and hidden products from WooCommerce query with pre_get_posts (plus equivalent filter for Ajax Load More)

wc-exclude-outofstock-query.php

<?php
/*
Solution based on a Stack Overflow answer by patrickzdb:
  https://stackoverflow.com/a/24514985/470480

Tax query solution for hidden products based on a WordPress Stack Exchange answer by kalle:
  https://wordpress.stackexchange.com/a/262628/19105
  
Please note:
  This file includes two functions/filters.
  
  The first is for vanilla WordPress pre_get_posts.
  If you do not use Ajax Load More, just copy the first function + action.
  
  The second is specific to Ajax Load More.
  If you use Ajax Load More, copy both functions + action/filter.
*/

/**
 * WooCommerce by default hides out of stock products AFTER the query.
 * This leaves gaps: If you have 16 products per page, but 1 of those products are out of stock, you only see 15.
 * This solution excludes out of stock products during the query, so you'll get the expected results even if some of the products are out of stock.
 *
 * @param $query
 *
 * @return WP_Query $query
 */
function rs_remove_out_of_stock_products_from_query( $query ) {
	if ( is_admin() ) return $query;
	
	// Only affect product archives
	// Consider using is_main_query(), although I wouldn't want out of stock products on secondary queries either!. 
	if ( !$query->is_post_type_archive('product') ) return $query;
	
	// Set up meta query
	$meta_query = array();
	
	// If a meta_query already exists, keep it -- require those condntions in addition to the stock query
	if ( !empty($query->get('meta_query')) ) {
		$meta_query = array(
			'relation' => 'AND',
			$query->get('meta_query')
		);
	}
	
	// Exclude out of stock products
	$meta_query[] = array(
		'key'       => '_stock_status',
		'value'     => 'outofstock',
		'compare'   => 'NOT IN'
	);
	
	$query->set('meta_query', $meta_query);
	
	return $query;
}
add_action( 'pre_get_posts', 'rs_remove_out_of_stock_products_from_query', 20 );


// AJAX LOAD MORE ONLY:

/**
 * (AJAX LOAD MORE VERSION OF rs_remove_out_of_stock_products_from_query)
 *
 * @param $args
 *
 * @return array $args
 */
function rs_remove_out_of_stock_products_from_alm( $args ) {
	if ( is_admin() ) return $args;
	
	$post_type = false;
	
	// Get post type, which may be an array or string.
	if ( !empty($args['post_type']) ) {
		if ( is_string($args['post_type']) ) $post_type = $args['post_type'];
		else if ( is_array($args['post_type']) && count($args['post_type']) == 1 ) $post_type = $args['post_type'][0];
	}
	
	// Only affect products
	if ( $post_type !== 'product' ) return $args;
	
	// Set up meta query
	$meta_query = array();
	
	// If a meta_query already exists, keep it -- require those condntions in addition to the stock query
	if ( !empty($args['meta_query']) ) {
		$meta_query = array(
			'relation' => 'AND',
			$args['meta_query']
		);
	}
	
	// Exclude out of stock products
	$meta_query[] = array(
		'key'       => '_stock_status',
		'value'     => 'outofstock',
		'compare'   => 'NOT IN'
	);
	
	$args['meta_query'] = $meta_query;
	
	return $args;
}
add_filter( 'alm_modify_query_args', 'rs_remove_out_of_stock_products_from_alm', 20 );

Updated to also exclude hidden products, as set in "Catalog Visibility" on the product page