It might happen that an exchange announces to delist a coin. You should check coinmarketcap.com, if there is another exchange and transfer them there. If there is no other exchange available you might have to enter them as "[OUT] Lost" if it is compliant with your local tax regulations.
You can also use the lost or stolen transaction type if the coins have been stolen, e.g. through a hack.
If there was an automatic conversion to another coin and a withdrawal (delisted coin) and deposit (converted coin) was imported, please change one transaction into a trade and delete the other transaction. If necessary, then work with the start date in the API job afterwards to prevent a re-import of the deleted transaction: How to Set a Start Date