Declaring has nothing to do with the USTRUCT or any other property specifiers. You declare the struct just by creating it (Struct declaration - cppreference.com)
The USTRUCT etc exposes the struct to unreal and helps with memory management etc, but the absence of USTRUCT / UCLASS / UFUNCTION doesn’t normally stop you using it as you’d expect within normal C++. So if you wanted to create a FMeatSaveGameArchive variable within a function, you can do so. If you want to pass it around to other functions either as a value type or reference then you can also do that. The only thing you might need to watch out for is memory management.
Basically, think of it like a normal member variable. You don’t have to use UPROPERTY(), but if you don’t then you just need to be a little more careful.