I was thinking about this.

In ACP we should have options what should happens with deleted topics, entries, photos etc. Now when moderator will delete topic, guest gets 404 error. In ACP we could have option that deleted things would be redirected to board index. When I click on link which belongs to deleted topic I should be redirected to board index rather than get 404 error.

I was thinking about this.

In ACP we should have options what should happens with deleted topics, entries, photos etc. Now when moderator will delete topic, guest gets 404 error. In ACP we could have option that deleted things would be redirected to board index. When I click on link which belongs to deleted topic I should be redirected to board index rather than get 404 error.

without 404 error, a searchbot wouldn't know that the page doesn't exist. It's not like it can figure out that it's being redirected to the index so that page didn't exist.

It would be redirected via 301. I think it's better solution than 404.

It's not, if you're talking about SEO. You should always send proper headers based on the response. If the topic doesn't exist, that should be a 404 error. A 301 is used when something has moved, and that topic has not moved to your board index page.