Yeah, I've never understood this. I've seen it now-and-then when installing WA for family members, but I (and they) have never, ever had to pay for it after using it for multiple years.
It's free since Facebook bought them. I've had Android friends mention payment screens are still on their WhatsApp though.
It was free before Facebook brought them too. It just nagged people to pay once a year, but if one didn't pay, it just kept working.