Speed and Nil Mode C in E
In the past ATC could ignore the nil-mode C 'cause ATC could assume that the tracks were clear of C airspace … if they were in C they would be in contact with ATC, identified and given a clearance (even without Mode-C) ... right?
Now, VFRs are allowed to be in E without talking to ATC … and if the Mode C is off or fails … ATC have no way of knowing what level they are (as highlighted by the B737/C421 incident, where the C421 was at F175 but no mode C!)
As a result of the B737/C421 event, controllers are passing more information on nil-mode C tracks … and the jets are taking avoiding action and diverting around the unidentified traffic.
For controllers, this increases workload and communication time in busy environments and takes away some of ATC's ability to assure separation (unpredictable diversions).
For the pilots, scanning for potential VFR traffic is an added distraction in a high workload period in an instrument-based environment.
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With regard to speeds in E …
ATC instructions do not overide the mandate limits defined for the airspace. So … an ATC instruction for MAX below A100 can't be issued if the aircraft could go into E.