I'd be very surprised if simply deleting your account prompted Coinbase (or any other exchange) or actually delete all the data they stored about you, including everything from KYC documents to IP address, activity, and trades.

If you live in the EU, you could put in a request under GDPR for them to delete your data, but it's anyone's guess as to whether or not they actually complied. Given stories like Coinbase illegally selling clients' data to third parties or Bitfinex and BitMEX ignoring laws forbidding them from allowing to trade in certain US states, exchanges don't really seem to care much for laws which impact their profits.

The only guaranteed way to keep your documents safe is to not distribute them across the internet.