It is ethical for large corporations to suddenly announce bankruptcy and leave hundreds of employees on the road? Or there should be a plan to help them?

Capitalism is either a profit or a loss. Each company expects bankruptcy at any moment. Although it has to be followed by preventive strategies and short and long-term employees do not even shocking decision to abandon them at any moment

It is not ethical. You should have inform about this to your employee give time to look for another job.

No, its not ethical. They should let the employees know few months in advance so that the employees can look for other jobs.

it must be make a plan and deal to give anything to emp. to help them