Am I correct that premiums on group term life insurance over $50,000 that have been waived because of the insured's disability is not taxable income?

I would treat it that way. Under a true disability waiver of premium the insurer is waiving the premium payments. The employer is not paying the premiums on behalf of the disabled employee. The employer may have a load in its group rates for this waiver, but they pay this for all employees.

But the amount that is imputed as income to the employee under Table I is not based on the employee's health or circumstances: it is the same for each member of the group who is insured, based upon his or her age within an age band. As such, if the employee is covered by insurance, even if nobody pays any premiums for or during the year, the excess over $50,000 creates taxable income for the employee.

Sorry

IRC Section 79 has an exception to the imputed income for an employee who is disabled here it is.
Exceptions
Subsection (a) shall not apply to -
(1)
the cost of group-term life insurance on the life of an individual which is provided under a policy carried directly or indirectly by an employer after such individual has terminated his employment with such employer and is disabled (within the meaning of section 72(m)(7)),

If the person meets the definition of disability in 72(M)(7) then there is no imputed income.

Thank you, Kip. I had missed that.