There seems to be no option to select blank / nothing in the drop down control, thus if you have a lookup field in your database which is optional there is no way of leaving it blank or deleting it after it has been created.
All other lookups elsewhere (Sharepoint, MS Access, etc) have an option to select blank. Can this be added into PowerApps?
You can achieve this scenario by using the combo box control instead of the drop down.
If you configure a combo box to disable searching (IsSearchable = false) and to be single-select (SelectMultiple = false) it should be fairly similar to a dropdown, with the advantage of being able to click on an "x" of the selected item pill to remove that item and have a blank selection.
We are planning to replace the dropdown implementation with that of a combo box, so that's the reason why this fix isn't being prioritized.