and what if a whole thread deleted? are we still get a notification about deleting the thread?
No. When the thread is deleted or trashed you won't be notified of anything. You only get notified when your post specifically gets deleted.
I am noticing my posts count constantly being lowered and lowered everyday. and I noticed the same thing in other's accounts too. (some of my friends also noticed the same)
That can happen when a thread in which you had posted gets trashed. You do not get notified when this happens. I generally recommend staying away from threads that break the rules, or have generally become worthless (full of rephrased signature spam).
I think Moderators removing old threads without any notification etc.
The age of a thread doesn't really matter if it breaks the rules.