If you want to see past birthdays or find upcoming birthdays on Facebook app 2021, just follow the steps below :
Open your Facebook app
Tap on the “search icon” at the top right corner.
Type “Upcoming birthdays” and tap the search button.
Some of your Facebook friends having birthday today will be displayed.
Tap on “view all”
Thereafter, you will see the complete list of birthdays including today’s birthdays, recent/past birthdays and upcoming birthdays on the Facebook mobile app.
I use the same trick to view yesterday birthdays of my Facebook friends.
I hope it works for you too.
So, if you are wondering what happened to birthdays on Facebook app, nothing really happened. It’s just currently hidden as a facebook shortcut on the facebook mobile app.