Carbon/soot certainly can build-up preventing the throttle plate from closing properly.  As the throttle plate is open when not powered it's quite easy to clean the housing. However I'm not so sure that the throttle plate not closing or sealing is the cause of the run-on.  My L322's 4.4 doesn't normally close the throttle on engine switch-off (it does now for 15 seconds via an add-on module in case of a failed primary turbo) and the engine requires fuel to run on. Fuel could be oil from excessive oil build-up in the bottom of the IC or from a dribbling injector.
What is the EGR fault code and what was the EGR work done previously?  
Why wouldn't the EGR issue be sorted before following any other hunches?
Edit: Prior to fitting my module to close the throttle the engine would be a little rough as compression stopped the crankshaft but since closing the throttle on switch-off the engine immediately becomes smooth, indicating that originally the throttle wasn't being closed.
I can't currently recall the cause but the TDV6 runs-on a little when a particular fault code exists as it was happening to my ex 3.0 D4 that a son now owns, but certainly not a throttle body issue.  Fix the problem causing the EGR code!
				
			 
			
		 
			
				
			
			
				MY21.5 L405 D350 Vogue SE with 19s.  Produce LLAMS for LR/RR, Jeep GC/Dodge Ram
VK2HFG and APRS W1 digi, RTK base station using LoRa
			
			
		 
	
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