Reviews can be deleted in the stores either by end users or by the stores themselves.
If the review is removed after it has been gathered to AppFollow - AppFollow does not automatically sync the status of the review and the review will still be visible in AppFollow reviews' feed.
However if you try to reply to such review from AppFollow - the review will be marked as Deleted. You may find such review using the filter Visibility --> Deleted. These reviews will not be reflected in reviews' feed without this filter. The replies for such deleted reviews do not use the replies' limit of your subscription.
The other option how reviews may be assigned with Deleted mark is if the Report a Concern has been submitted via AppFollow and the review has been removed after this request.