Most forums wouldn't waste hard disk space on such a forum. It's easier to delete. If you know SQL with the right query you can even erase an entire user, their every post deleted across all forums. Basically software like PhPBB, SMF (which this site runs) and others are interfaces over a database, usually an old fashioned SQL database as most forums aren't high traffic enough to benefit from the scalability of NoSQL, whereas they do benefit from referential integrity, which SQL provides. It's not hard to write your own custom forum with a minimalist feature set; I know of one website that was run by a couple of nerds at the CS lab at the Univerisity of Lulea, one of the best engineering schools in Scandinavia, who ran a huge forum using software and hardware they built themselves. And the forum was only a tiny part of their website, which generated substantial ad revenue from its photo DB (it was an aviation website) and was often used as a source of photos by the media when a crash or other aviation related story happened. And the photographers got royalties with the site taking a cut. It's still in business but has changed owners a few times, but it's still running the same increasingly dated software (which has been running with modifications and upgrades since the 1990s; it's years older than Twitter and a relic of the dotcom bubble), as replacing it would be a huge and possibly unsuccessful software engineering task.
So I for one am, reflecting my Orthodoxy as a Christian and as a UNIX programmer, skeptical of purgatory. Missing threads are surely cast into the lake of fire: hard disk space marked "unused", where they will suffer the torture of being overwritten with other data until the end of time. The NSA according to rumor has the ability to recover data that's been repeatedly overwritten; I suspect this is exaggerated,mbut it does suggest an extended afterlife for deleted files as the byte patterns slowly degrade and succumb to corruption.