Hook documentation is on the wiki. https://github.com/NodeBB/NodeBB/wiki/Plugin-Hooks
Although it might not be 100% up to date.
It would be nice to have a feature where you can set an automatic purge job that purges old topics that are x (setting in ACP) days old. Even better, topics that do not have any activity for x number of days. This should help clean up larger forums. Should this be a plugin or a core feature?
I wouldn't mind an archival feature -- but I think plugin.
This, but most of my posts are links to old mixes, so I don't want them to vanish.
You should check out the archiver plugin. Newly refactored for NodeBB 0.6.x as of two days ago!