Can someone tell me if there is a required limit of time in which a former employer must provide written notification of a deferred vested pension benefit following the employment termination date?

Notification should be by the time of filing the Form 5500 for the plan year following the plan year in which the severance of employment occurred. Such participant should be included on the Schedule SSA of that filing, unless the benefit has commenced by that time. BTW, if you include them on that (or any) SSA, and later make a complete distribution, it is OK (even advisable) to use the next SSA to remove them.