I've been upgrading our firmware to use the v19 release of tracker-edge.
regCommandCallback has been removed in favour of registerCommand, but I can only find documentation for the old method (Tracker Edge API reference | Firmware | Particle)
Yes, you should use registerCommand instead.
int registerCommand(const char *name, std::function<int(JSONValue *)> handler);
It will only be called on success, and otherwise it works the same as a success handler with the default parameters with the old version.
It's now documented here in the Tracker Edge API reference.