Will the app still be supported?
The app will not receive any more updates. Coinbase could make the decision to drop support for the existing iOS app users at their discretion.
Will the app be susceptible to vulnerabilities?
Coinbase does not store private keys on the mobile device so no one can spend your funds without verifying with coinbase's server. I can't remember if the iOS app has 2 factor authentication. If it does you should be safe from future vulnerabilities.
Should iOS users abandon Coinbase app?
That decision is left to the users.