Although I am not certain, here’s my attempt at making sense of the situation.
The change is not listed in the Change Logs for IFC22.214.171.124, so I can’t really find out why is was deleted or what replaced it.
Even when looking through IFC126.96.36.199, I am not sure where you would even store such information. Perhaps someone else here may know something.
From the looks of it,
\__ExternalSurfaceArea are part of the IfcComplexProperty that is named
ExternalSurface, as is mentioned in the definition:
External surface area quantity. Several complex properties of this type may be attached to this Pset, one for each external subsurface with different properties
I think this is the reason for the
\__ before the properties.