Jordan Kahn has details on how Apple can recover data from a T2-enabled Mac:
Apple notes that the process for data transfer for Macs with the T2 chip will be used when its repair staff are presented with a customer’s machine in need of a logic board repair and when the logic board is “partially functional.” The process also requires that the system can be powered on.
To complete the process, Apple’s repair staff will use a Thunderbolt (USB-C) to Thunderbolt 3 (USB-C) or Thunderbolt 3 (USB-C) to USB-A cable and another host computer as well as an external hard drive where the data will be transferred. From there the machines can be put into DFU mode and the data recovery process can be initiated from within Apple’s internal diagnostics tool.
I really love my iMac Pro and new notebook, but this makes my heart hurt for the days where I could just pop a drive out of a MacBook with three screws.