Whenever you destroy a droplet it's IP address remains reserved to your account for up to a few days. The next droplet you created in the same region will inherit the IP address.
Where does DO explicitly document same?
This implied it is very short-term, speed is of the essence to get it back:
https://www.digitalocean.com/community/tutorials/how-to-downgrade-digitalocean-droplets
Step Five — Destroy the Larger Droplet to Release the IP
"... next step will almost always re-allocate that same IP address to the new and final Droplet. You should do the next step within a few minutes of completing this one..."
Step Six - Create the Final Smaller Droplet from a Snapshot
"... new smaller Droplet should have the same public IP address and the same files and data as the previously destroyed larger Droplet..."
