Every week, spam robots signup a thousand or more members, but our defensive measures keep almost all of them from posting.
Eventually, we clean up and remove members that haven't logged in for a while.
We do have the ability now to only remove old members who have no posts or only a few, so hopefully active members will not get removed if they take a 6 month break.
Its time to clean up again, but it brings the system to its knees to do tens of thousand's at once, so it must be done in small chunks.
We cannot retrieve them, but you might be able to signup again with the same user name, I've never tried it.
If a user is gone, he is gone, but the posts may remain.