I thought I noticed this last year, so I used the -18°C morning yesterday to confirm it - the allowed regenerative braking power (at least in my 2016 SL) does not seem to account for power used by climate control. I charged the car to 100%, set the heat at 20°C and the fan to 4, resulting in just over 3 kW being used for heating according to the energy display, with another 600 W or so for the fan, seat heaters and other accessories. When slowing down (either D mode or B mode) or braking under these conditions, only the tiniest blue sliver appeared on the motor power dial on the display, no more than 1.5 kW. I would have expected at least 4 kW of regen to be available under these conditions, plus whatever small amount can be safely put back into the battery when it is cold and near fully charged.
This behavior is also consistent with the state of charge ending up at 90% after a full preheat cycle following a full charge, which happens when it is very cold even though the car is still plugged in. I expected some battery drain as I only have a 16 A EVSE, but losing 10% in 30 minutes suggests that less than the full 3.6 kW available from the EVSE is actually being used, as if the charger is ignoring the power being drawn by the heater and is applying to the same low charge rate it would use for a cold battery that is almost fully charged in the absence of any power draw.