In C++ you add “AdvancedDisplay” specifier to UPROPERTY() and yes “|” creates subcategories if you use it in “Category” specifier. I guess this does not work just by typing in category Blueprint, i don’t think you can do dropdown in blueprint either. Lot of cosmetic features are exclusive to C++, you might consider posing feature request in feedback forum, other then that you need to use C++. Here you got list of specifier you can use in UPROPERTY:
And UFUNCTION: