I looked it up in the relevant reg defining wages, 26 CFR §31.3401(a)-1, and it seems to say that “pensions and annuities” are wages:
“(b) Certain specific items - (1) Pensions and retirement pay. (i) In general, pensions and retired pay are wages subject to withholding. However, no withholding is required with respect to amounts paid to an employee upon retirement . . . "
