Hello. Can you advise on the best course of action if we remove entire sections with products from our website and will no longer sell these items?
Should we give a 404 error and lose the link, or redirect to the main page since the section no longer exists and may cause negative behavior from users?

The obvious solution would be to redirect the link to the main page. This way, users won't visit an error page in vain and will be directed to the main page instead. Imagine you click on a search result that leads to a 404 page.
Would you continue exploring the website or close the tab? Most users would close the tab and move on to the next search result.

By implementing a redirect, you can avoid negative feedback from users who encounter multiple 404 pages. 301 redirects are commonplace, and by doing so, you won't have to keep track of sections that have suddenly disappeared from your website. However, it could make it difficult to track user behavior and where they are coming from. It may be best to handle 404 errors on a case-by-case basis by finding out the source of the error and implementing a redirect to the correct page.

This task can be assigned to SEO specialists. If redirecting to the main page is deemed appropriate for visitors who previously accessed addresses such as http://site/forum, then this can be implemented. Pages with redirects will eventually disappear from search results.