Good question, here is hoping for a definative answer as I don't know. The switch obviously affects decel cut-off by indentifying closed throttle, but there already was a closed throttle switch in earlier 103 KE systems and it continues after the introduction of the decel idle switch in about 88.
I can tell you it does affect idle. Its my impression that it (along with the regular throttle switch) participates in the off idle control state. This is the strategy the valve takes when the speed is "high" but the throttle is open. Normally the valve would close at "high " speed but this is not what you want to happen when idle is not the point.
Bosch Master, ASE Master, L1
33 years MB technician