Doug wrote:
Misposted a bit, radar coverage is not mandatory for IFR flight but ATC
clearance is and it USUALLY includes radar coverage.

ATC clearance is mandatory for IFR flight IN CONTROLLED AIRSPACE.
It is neither necessary nor available outside.

Radar coverage while pretty pervasive isn't a requirement for nor
a guarantee of an IFR clearance availability. Nearly all flights
begin and end outside of radar coverage, some outside of controlled
airspace.