I did not know this. Source/docs?
As a note: you'll have to add a valid payment method on DigitalOcean to activate your account, but once you're in it's free to use all the DNS features, and you can automate via the API, docs here: https://developers.digitalocean.com/documentation/v2/#domain...
AFAIK, there is no plans to move to paid service. It makes sense to make DNS management easier for developers if you have your applications or websites running on our droplets.