Woocommerce – exclude only older out-of-stock items

Most information necessary can be found at the Code Reference: WP_Query | Class . Otherwise useful resource Theme Handbook: Conditional Tags. Below code contains some clarifying comments. Please note, this is untested code, so no guarantees, but it should get you going.

add_action( 'woocommerce_product_query', 'custom_woocommerce_product_query' );
function custom_woocommerce_product_query( $q ) {
  if ( ! is_admin() ) {   
    //query for the out of stock items we want to exclude
    $oos_query = new WP_Query( [ 
      // items must be older than 1 month
      'date_query' => [ [ 
        'column' => 'post_date', 
        'before' => '1 month ago' 
      ], ],
      //only out of stock items...
      'meta_query' => [ [ 
        'key' => '_stock_status',
        'value' => 'outofstock',
        'compare' => '=',
       ], ],
      //we want products, nothing else
      'post_type' => 'product',
      //we want them all
      'posts_per_page' => -1,
      //the ids are enough, no need to get more data 
      'fields' => 'ids',
    ] );
    //getting array of ids from object
    $exclude_ids = $oos_query->posts;

    //excluding the above queried products from woocommerce's product query
    $q->set( 'post__not_in', $exclude_ids );
  }
}