Originally Posted by robwod

Personally, I would not discard the reviews (some folks would just delete them). It is already indexed and/or indexable text content. I would consider updating each review with something simple like, "Editor's Note, this site is no longer available. However, click here to see other sites within this same niche" and Link it to a niche tag on the same site. Keeps things relevant and helps update old content to point to new content.

There's other options, of course, but that's one to consider nonetheless.