The " 7.0 Upgrade Assistant" was complaining about the
.tasks index which was created in an older version 5.4 or around there. I assumed it was one of those system indexes that, if deleted, would be automatically recreated ES needed it. I've gone ahead and deleted the .tasks index and everything seems to work ok.
Was my assumption correct? Will the .tasks index be recreated if it's ever required?
ES version 6.6.1