We sign contracts with non-poaching elements in them all the time because the companies we look at are afraid that we will use our inside knowledge to go after their best talent. But I suspect that just the basic protections an employee has under employment law would be enough to stop a company from trying to enforce this (we have a term for this in Dutch which is called 'broodroof' but I don't know any good English translation).

That said: for Execs and consultants it is a different matter. But they are not rank-and-file employees.