I have learned from experience that if you leave a 404, that page can sit in googles index for months, if not years. Google defines 404 pages as "missing", which means they will keep it in the index and keep checking back. They give quite a long grace period.
Google defines 410 as "Gone", which tells them the page will not be returning, and its absence is intentional. This keeps forum SEO listings very clean, and speeds up the process of URL removal.
The option to define 404 or 410 (dropdown menu option in the Admin maybe?) for deleted threads, deleted profile pages/ deleted users, deleted tag pages, and anything else that can be intentionally deleted (on a page level) would be outstanding.
As it stands, there are no Addons for this, and I may need to pay my guy to create it.
By the way - very thankful the XenForo folks knew to at least 404 that content! vBulletin leaves it as a 301, which is really bad! It results in "Soft 404's" which Google actively penalizes for (unlike standard 404s).
The ability to 301 redirect a deleted thread would also be incredible. Sending it to an existing thread that has better content, or even if the forum owner wants to redirect it to the forum home page.