I don't think that the rule is new. I think that some plan administrators have been allowing alternative methods of specifying withholding and treating that equivalent to submitting a W4-P. However, I do believe that the IRS mandates the use of an actual Form W4-P to increase the default federal tax withholding and the IRS might simply be increasing enforcement of the requirement. (Because an RMD is ineligible for rollover, the default tax withholding is 10%.) There is likely a different but similar form for state tax withholding.
I suspect that the IRS mandates the use of the actual form because the form also includes the instructions which provide important information to the individual. The rules are different for IRA distributions where there is no IRS requirement to use a particular form.
