Are retroactive payments made back to the Required Beginning Date under a defined contribution plan fully taxable in the year paid or the year they should have been paid?

Since pension distributions are taxable in the year of actual receipt, I don't think it makes a difference if a pension distribution should have been made in a prior year; I believe that the money should be taxable in the year of actual receipt.

For regular income tax, individual taxpayers are cash basis, and the distribution is from a qualified plan, so the amounts would be taxable when paid.
However, the excise tax on failure to make minimum distributions is another issue. One could file with the IRS, requesting relief.