Reverse DNS for Foating IPv4 and IPv6 on DigitalOcean-supplied or Customer's Domains
DigitalOcean owns the IP blocks from which Floating IP addresses are allocated to droplets. It would be a benefit to the customer to automatically assign Reverse DNS records to the Floating IP addresses handed out. Mail Exchange servers, and any secure server are usually required to have Reverse DNS match Forward DNS, for security reasons.
Steve Summers commented
It is pretty much a no-brainer, since DO owns the Foating IP blocks. Any company's public-facing MX server, or other public server which requires secure encryption and higher levels of trust, is expected to have a domain name with an IP address that matches forwards and reverse. This is just one of the things needing to be in place to establish trust.
Since DO owns the Floating IP blocks, it would make sense that DO would offer some mechanism, either automatic, or otherwise, to allow for the reverse DNS entries.
If DO charges for Domain Name management, even if the customer had their own DNS handled elsewhere, I would think DO could charge something for handling the reverse DNS services for the domain name pointing to their public IP address.