Yes, that is what a topic on Stack Exchange suggests. It is easy and harmless to add. But I have doubts on effectiveness because it was apparently never needed before, and when I build it locally, I don’t get this error.
Explicitly including the ExternalProject module is only required with Slicer 4.8.1. Since r26984 it is included through UseSlicer.cmake. This is why it was removed in SlicerRt/SlicerRT@6495332