It seems he favored the idea. Mario Rizzo directs me to his letter to James Meade, circa 1942:
am converted to your proposal…for varying rates of contributions in
good and bad times. (June 16, 1942). Keynes, Collected Writings, vol.
27, p. 208.
are able to show fluctuations in income of an order of magnitude which
is significant in the context… So far as employees are concerned,
reductions in contributions are more likely to lead to increased
expenditure as compared with saving than a reduction in income tax
would, and are free from the objection to a reduction in income tax
that the wealthier classes would benefit disproportionately. At the
same time, the reduction to employers, operating as a mitigation of the
costs of production, will come in particularly helpfully in bad times. (July 1, 1942). Keynes, Collected Writings, vol. 27, p. 218.
Will today's Keynesians follow suit?