As you must already know that whenever you add a new friend on Facebook, you get a notification; however, this is not the case when someone unfriends you.
Facebook has not introduced such a feature to date through which you can be notified if someone has unfriended you on Facebook. You can only know this by analyzing and observing.
To know if someone has unfriended you on Facebook: you need to go to your Facebook friend list and enter their name in the search bar. If their account does not come up when you search their name in your friend list, then it means that person has unfriended you on Facebook.
Another way to know if someone has unfriended you on Facebook is that you won’t be able to see their posts on your feed. If you notice that a person has not recently liked your posts, then it could be a sign that he or she has unfriended you on Facebook.