The MFI (Made for iPhone) program is today a significant revenue generator, allowing Apple to take a commission on every certified iPhone accessory. if Apple wish to continue licensing accessories for money in this way they will likely need some way of identifying approved accessories as they can with Lightning today.
The MFI program largely makes money because Apple control the lightning connector interface and can thus be the gatekeeper. While I don't like it personally, it does make some sense to me the mfi program might want this feature for a USB-C iPhone and future iPad models.
Like others, I suspect it would be data access only limited to the certified devices - any old USB-C cable will likely still work to charge. This likely satisfies the EU charging law changes and lets them keep much of the MFI revenues.
That the MFI program exists is frankly evidence enough for me to believe Apple would do this - they sell many more iPhones than iPads, and the change can be incorporated into future iPad models.