1. DNS/floating_domains/DigitalOcean

1.1. what happens

Returning to DigitalOcean, 
the answer to the question “what happens when I delete my domain from DigitalOcean but forget to change the nameservers?” becomes clear.

If you delete the domain from your account 
anyone can immediately re-add it to their own account without any verification of ownership and take it over.

1.2. DigitalOcean’s Response

Matthew,

Thank you for sending this in. This is a known workflow within our platform. 
We are committed to always improving our customer’s experience and 
have been examining ways of minimizing the type of behavior you are describing.

    Regards,
    Nick
    —
    Nicolas [REDACTED] security@digitalocean.com

https://www.digitalocean.com/docs/networking/dns/