Can anyone verify if the Annual Open Enrollment period is mandated by state or federal regulations, or is it up to the individual carrier to offer?

No federal requirement for an annual open enrollment. HIPAA requires Special Enrollment provisions.

Although there are no Federal requirements, OE is usually done annually for other reasons such as to allow changes in coverage (whether plan selection or dependent coverage), changes in Salary Reduction Agreements, changes in FSA etc election. Some also do it so as to observe state payroll and labor laws regarding salary reductions/deductions.