The software doesn't seem to provide a good solution to resolve soft 404 errors for search engines.

Google Webmaster tools finds "soft 404 errors" which it considers 'problematic':
https://support.google.com/webmasters/answer/181708?hl=en

When a page doesn't exist 'for the world out there' (particularly a bot), it should return a 404 error!
However, in phpBB, the software doesn't. If a user has been deleted or if a thread has been moved to a section of the board that hides it from the outside world, it returns a code 200 "ok" and redirects the bot to another page, but doesn't display actual content which is interpreted as a structural problem of the whole website.

It think this should be fixed - VBulletin, which once also had this problem as my web search turned out - did that already. phpBB could actually redirect the user to some other page but should definitely return

{{header("HTTP/1.0 404 Not Found");
}}
to bots.