I am not sure it is ever used, at least not now. Using VA 1538 there are 4 refactoring entries for C++, and "Refactor Extract Method" also has a $MethodBody$ entry.
From memory "Refactor Create Implementation" used to be used by both Create Implementation and Extract Method, but now there are two separate rules for these two operations.
it seems Create Implementation can be used for "Move Implementation" and "Extract Method", and when these are happening $MethodBody$ is required. the rest of the time it is just ignored.
Autotext for Create Implementation is used also by Move Implementation to Source File so $MethodBody$ needs to remain where it is.
As well, the autotext entry for Create Implementation is used by Extract Method when programming in pure C. (The entry for Extract Method in not used.) $MethodBody$ tells Extract Method where to place your implementation.
Leave $MethodBody$ where it is and you'll be fine. It gets resolved to nothing when not needed.