Not supported. Not nice. π
I realize there can be some unmaintained distros which do not have ed25519 support in openssh, but the feature was added to git version of openssh almost two years ago.
I'm glad to say that support for ed25519 SSH keys has been rolled out to our control panel.
While you can now store ed25519 public keys and add them to Droplets on create, using ed25519 keys is still dependent on distributions shipping OpenSSH version 6.5 or greater. While support is present in our default Ubuntu 14.04 image, some older distributions (e.g. CentOS 6 and Ubuntu 12.04) do not ship with support for ed25519.