The registry key for disabling path length limitation is off by default. Since this patch, the setting becomes visible for Windows versions on which the limit can be disabled. But user has to explicitly opt in, at least for now. Microsoft might change that in the future, once all the relevant tools get upgraded to work without issue with longer paths.
Without this option, there was no way to build ITK and projects which have it in its super-build (e.g. Slicer) in a longer path.