Is implementing the AppTrackingTransparency prompt mandatory for use of Branch?

No, it is not necessary to implement the AppTrackingTransparency prompt in order to use Branch: there are many valid Branch use cases that do not fall within the scope of Apple's policies on 'tracking', including deep linking and measurement of owned/organic channels.

For more information, please see these FAQ items:

• What counts as an 'ad' and 'tracking' under Apple's policy language?
• Are owned channels under the scope of Apple's new policy?
• Are organic/earned channels under the scope of Apple's new policy?
• Does Apple's new tracking policy block deep linking?

If you choose to show the ATT modal, you will simply continue receive device-level data from Branch for ad conversions when the user opts in on both ends.