Does anyone know if there is a requirement for identifying multi-wire branch circuits in the panel. If not it seems to me there should be. It can be very difficult for the next electrician to properly modify breaker spaces if need be without knowing which neutral goes with which hots.

No requirement. That's just one of the challenges of being an electrician, I guess.

I understand the concern. As a regular practice, on raceway systems, I group the neutral with its related phase legs and use a wire tie or, at minimum, tape to identify the group, close to the raceway entry to the panel (assuming more than one neutral is in the raceway). If I have to verify neutral - phase leg combinations on an existing panel as part of the changes I'm doing, again, I'll mark the multiwire group with wire tie, etc., so as not to have to duplicate my effort the next time.

Al

I feel it's always a good idea to put a wire number marker on both the phase conductor and the associated neutral. If the neutral is shared, put both or 3 numbers on it. This is good practice especially for conduit systems, but also for cables (NM, MC) as well. You never know when you may have to move a wire around, and why not make it as easy as possible to identify it?