AFAIK entries beyond a certain date are simply dropped. That's what happens to my Inbox when i wait too long to check posts and messages or hesitate to reply to a discussion.
In my Inbox, tit still happens with items older than 6 months: Maximum age of Inbox items
Items older than 4 months should reappear by either navigating to the bottom of the list (automatically expanded with a limited number of pages) or by clicking the "show more" bar.
You can also unmark the "Unread only" checkbox to reduce the length of the list, but that will only show items between 4 and 6 months old if you didn't already read.them before.
