I usually just go back via the page # tabs if it has moved back a page or 2 , or remember the forum it was in and find it that way..

It shouldn't actually disappear, but maybe needs to be found again another way...
If you can recall the forum & thread title that will help you to find it.

I don't know if there is a cut of # of subscriptions - but you can go into your user CP and remove the oldest ones.