Moreover, if you were using Coinbase for crypto-only transactions and never passed verification, your account will now be locked and you will be required to verify identity:
There is a workaround though – normally if you just delete your account it should forcefully clear your balance.
However, deleting your account will still require a phone number from you.
Go to Settings -> Security -> Delete Account (the button is located at the bottom of the page). Click that:
If you have remaining balance in any cryptocurrency – you will be required to enter your phone number to proceed with the withdrawal:
Considering that Coinbase works with blockchain tracing companies and their contractors are often selling data, if you want to at least keep a degree of anonymity entering your phone is not a good idea. Obviously if you had a large amount of coin on Coinbase you’d have to do it to save your funds.