Open Neutral Service Detector/Disconnector
In the USA UL and ISO requires that surge protection devices (SPD) open neutral with the same built-in disconnect switch that opens the mains power. There is concern that neutral may have a voltage above 30 VAC if the SPD is far from the service entrance panel. In other words it is located at a distant sub-feed panel for local surge protection.

You would never disconnect neutral at the service panel unless the same switch disconnected both hot wires, or 3 hot wires in a 3-phase system. The only reason for doing this is if you have a powerful backup 240 VAC split-phase generator and your power utility company does not want neutral to be shared with another power source. Since neutral is grounded at the service panel this could lead to ground loops that have current in them if the generator is not grounded locally.

The same rules apply to solar power that is powerful enough to run the house from a charged battery bank and huge DC to AC power inverters. Even auto-switch over relays must disconnect neutral as well as the hot wires.

Your last paragraph is a bit misleading as many 240 VAC appliances use just the 240 VAC hot wires, and do not use neutral. They do use a ground wire of course, and it has to be the same gauge as the hot wires.

Electric dryers, Air Conditioners and electric water heaters and electric ranges/ovens do not normally have a neutral return as they do not use it. The big 3-prong plug for them is just the two hot wires and a ground. By UL standards any 120/240 VAC powered device with a metal chassis must have an Earth ground.

EDIT: There are GFCI and other devices to detect an imbalance in current in the sense that the neutral should have the same current as the hot wire. If not then a device becomes a shock hazard. Some breakers that feed bathrooms and kitchens also have GFCI circuits built in as well as arc detection, often used on older homes where the wire insulation was not of good quality.

At this time I know of no device to be installed at the service entrance panel to detect if the neutral is cut outside of the house. I had that happen to my house and many homes in our neighborhood due to tree roots cutting underground cables. Our neutral was cut so one phase went low while another went up to 200 VAC, burning out some ceiling lights.

The power utility came in and re-wired all the homes in this division of 57 homes by running new power feeds through thick PVC pipe.

A simple "imbalance" detector could have tripped and cut our power before things burned out, but that was 20 years ago. Nothing has changed. There are industrial 3 phase motors with a cut-off relay if a phase fails. Sad to say but for most any home or business it would be a custom built add-on.