It is required to have a radio ..
only in some operations so I agree, AirlinePilot, for private and aerial work but CAO 20.18 is only operational requirements. There is another set of stuff required for airworthiness certification for which you need to read the POH/AFM or even the Type Certificate Data Sheet to check what is mandatory.
eg from my copy of Report
VB-880, POH for the Warrior:
propeller is required but not a spinner
nose wheel is required but the cigar lighter is not
stall warning is required but carb ice detector is not
and in one of the regs it states that maintenance and inspections must be certified
and in another it states that pilots can do a daily inspection
so the pilot who does the DI must sign to certify that