is it safe to sell broken iPad pro for confidential data

Ideally, prior to disposal or sending your iPad for repair, you would prepare it properly:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Get your iPhone, iPad, or iPod touch ready for service - Apple Support

Unless the iPad can be successfully powered-on, you'll not be able to erase the iPad. As described this isn't possible - but this situation may not be the bleak situation that you might anticipate...

Nobody, not even Apple, can gain access to your iPad without successfully entering the correct device Passcode. iOS/iPadOS is architecturally designed to protect the owners data.  All locally stored data is encrypted; by design, the only copy of the encryption keys necessary to access local data are held within the Secure Enclave - the device security chip. The device Passcode unlocks the Secure Enclave, which in turn releases the encryption keys to the Operating System while the device remains unlocked. Even if you don't (or can't) properly erase the iPad, unless the iPad is fully functional and the Passcode is known, your data cannot be accessed.

Nobody can gain access to your iPad without successfully entering the correct device Passcode - and for this, there are a very limited number of tries available before the iPad will automatically become disabled. As such a brute-force attempt to access the iPad, by guessing the correct passcode, is highly unlikely to be successful.

If you are concerned for the security of your locally stored data, you might be best advised to return the iPad to Apple for responsible recycling.