Prior to creating a droplet one must first choose a hostname for said droplet. Is this hostname reachable over DNS (i.e. is it announced via ns1/ns2/ns3.digitalocean.com) such that I can SSH into myhostname.digitalocean.com?
It is reachable if you enter example.com as a hostname and you have example.com registered with your preferred registrar (Namecheap, GoDaddy, etc.), and it is pointed to the Droplet.
AFAIK, there is no way to access your droplet from a hostname like myhostname.digitalocean.com because DigitalOcean does not provide subdomains like that. If you don’t have a domain you can always SSH into your Droplet’s IP.
It works for me. I use my droplet as a workstation (SSH+tmux+vim).
It’s only “reachable” if you have your domain registrar point your domain’s name servers to DO’s name servers (as you seem to have already done).