The challenge here is that there are people out there who just do not understand the inherent value of this kind of old content. If any company exists for long enough eventually some of those people will cycle into positions of decision-making authority where they get to "save costs" or "clean things up" by delete-hammering some invaluable artifact like this one.
The correct way to handle this is to add a BIG banner on top of older content like this warning "This post is from 12 years ago, and may no longer reflect the most recent version of our software".
Flattening a bunch of dynamic pages to static HTML can also be useful, if the concern is maintaining old forum software.