You're right Mike, decel conditions cause misfiring and huge HC and CO, but the cure was the decel valve, which in most SU and Stromberg carbs is a spring loaded valve in the throttle plate, that admitted enough mixture on decel to provide a burnable mixture. On other carbs it was a diaphragm that prevented rapid return to the idle stop.
Part of the reason for the various inefficient methods was the legislation that measured the emission components in units that weren't compatible. Some were in percentages of total exhaust gas flow, and others in grams per mile, which did not consider total flow. Nox was a difficult gas to elimnate, and measured as a percentage as I recall.