And this was never really enforced. I don't know anyone who paid for it.
I remember when I paid the $0.99 after the first year a friend told me "if you uninstall the app and reinstall it, you get another year for free".
My father was one of its earliest adopters, even before I was. He paid for it. I joined maybe sometime in 2012 or 2013. WhatsApp kept renewing my subscription for free. Also, fun fact, my first version of WhatsApp was on a Nokia phone... With the Symbian OS.