Support confirmed forced archival is a new "feature" for cloud hosted sites with no options on what areas to archive. Apparently this is undocumented and set to 3 years (1to5 is range). Claim it is being looked at. Not having advance notice of this change is in pretty poor form.
For sites with reference type forums (food, wine, books, travel, technical advice) this is pretty fatal as content over 5 years is no longer searched, and pretty useless. We have low density forums with over 10 years of history and the content there is not time sensitive.
clearvision's post in No unarchive option... was marked as the answer