Even running your own server doesn't guarantuee 100% reliability on any but the messages fed directly to your server. Messages sent to a newsgroup you carry but via a another news server may never reach you.
NodeReaper posts are actually still there - you can Visit Reaped Nodes and read the previous content. It is removed from immediate view, but not lost.
The last one is a good point - one might argue that PerlMonks could keep a full history of edits, but then one might also argue that archivers can ignore the headers requesting the message to be discarded.
Makeshifts last the longest.