Yes links to the old form are 100% stable. The old forum as been archived to static html, this makes changing the old forum very difficuit which also means that the links will be even more stable now than when the old forum was active. The old forum html files are all on github if you want to search / host locally: GitHub - openenergymonitor/forum-archive: Archive of old OpenEnergyMonitor forums (May 2016) https://openenergymonitor.org/forum-archive
All file attachments have been archived (none have been lost): forum-archive/sites/default/files at master · openenergymonitor/forum-archive · GitHub
However, if an file was ‘attached’ to a post but not linked into the post body then the link to this attachment won’t show in the static html. I am working on fixing this.
I will let @TrystanLea advise on where the best place for the virtual feed docs to go. I agree, that post by @nchaveiro on Virtual Feeds is very good and should be in the latest documentation.