i have out of stock products appearing in my shop/categories/search results pagese.
how can i show them at the bottom of the sorting? in shop/categories and search? (i have a contact form in them so they are useful)
You cannot, unless you accept to loose your current sort order (like relevancy, or most popular).
You could add the instock/outofstock custom field as a filter: your visitors can decide to only see stocks, only see out of stocks, or both.
You can also set the “instock” filter value as defaulted: your visitors will only see stocks, unless they decide to deselect the filter to see all products.