If you destroy your droplet all data is erased.

However, you also want to make sure that you have no snapshots, or backups of the droplets under the images tab in your control panel as the droplet can be recreated from a snapshot. If you have those delete them as well.

Lastly, if you used Block Storage and attached a volume to your droplet you will again need to delete that separately as well as any associated images/snapshots that you may have taken.