Some reddit users are egregious about it, installing scripts that overwrite their comments after x amount of time, seemingly oblivious to the fact that every edit on reddit can be found through archival tools. The solution is to take better care to not conflate your anonymous online persona to your real life persona—just don't post identifying information publicly online and you will be head and shoulders above many users on the internet in terms of privacy. There's no need to purge the internet of it's collective knowledge and history.
We are very lucky to have the wayback machine preserving this stuff from dissapearing into the void, but it doesn't cache everything on the internet, especially if that forum I visited had shut down and became impossible to find in a search result.
Side note: Is there an extension or bookmarklet available to automatically pull a web archive?