Because forums software changed, some external links are not functional.
It is possible to manually navigate to old content by modifying the old link:Before:
https://forums.counterparty.co/index.php?topic=253.new#new
After (current forum):
https://forums.counterparty.io/discussion/253
There seems to be one problem, though - it in above example "discussion" is the name of a section and I'm not sure how that can be derived from the old link.