They are, but cars will function without them being used.
I don't think it is mandatory, but it counts in the EuroNCAP score, and since it is one of the easiest safety feature to implement, they all have it.
My first car didn't have a seatbelt reminder, and it needed a bit of time to warm up, so I got in the habit of starting it and then buckling, and 20 years of driving with seatbelt reminders hasn't trained me to switch the order.