okay, okay... I'm currently writing a plugin that can at least restore symmetry after editing a side (without changing point order, UVs, selections, morphs, or weights). That's not quite the same as tools that obey symmetry by themselves, but may help the user until MAXON includes full symmetry with their C4D functionality.
Yes, there used to be some tools, but symmetry needs to be quite deeply ingrained in a system to work without wrecking anything else, so they may have had restrictions. (The C4D API has some messaging that is triggered by changing geometry, but that is quite a beast.) Also, I think none of these tools have been adapted to R23 (yet?).
If you know of such tools, please add them to this thread as useful links.