Once a user opts-in to device-level tracking, can it be connected to that user’s previous activity before opt-in?

Branch will not be retroactively updating events that occur prior to opt-in. You may be able to analyze these events in your own BI systems by using other identifiers such as IDFVs.

For a full walkthrough of the device-level ads analytics changes Branch is making for iOS 14.5, please read this blog post.