As other users mentioned, the post should be gone with deleted threads and that's why I didn't get the personal message by the mods.
It has to be true, and that's how it has always been when the thread was deleted by mod.
There's nothing we can do about it so far, but avoiding newbie thread especially pointless or uninteresting thread is a great way to maintain your post count.
I have some questions now about the activity, Is that number affected by the deleted posts too? or in some scenarios a user can have 900 posts and 1000 activity?
Same by DdmrDdmr response, the activity count will be affected by significant reduction in the number of posts. But for relatively low numbers, it won't matter.
You can also read more details here:
https://bitcointalk.org/index.php?topic=2766177.0Can activity or my rank decrease?
Yes. It can only decrease if you have all the posts you made from a particular period deleted or the threads you posted in trashcanned. If you do it's possible to drop rank until you have enough activity again.