"TCJA amended section 3405(a)(4) ... to provide [that], in the case of any periodic payment with respect to which a withholding certificate is not in effect, the amount withheld from the periodic payment will be 'determined under rules prescribed by the Secretary.' ... This proposed Section 31.3405(a)-1 provides a flexible and administrable rule that leaves the communication and mechanical details of the default rate of withholding on periodic payments to be provided in applicable forms, instructions, publications, and other guidance.... While this proposed regulation would update certain Q&As in Section 35.3405-1T, it would not update all of the Q&As, including several Q&As that do not reflect legislative changes that became effective after the publication of Section 35.3405-1T.... This regulation is proposed to apply to periodic payments made after December 31, 2020."