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:

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.

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