Dead Spots or places where ATC or "Radio" just doesn't answer?
I'd say some of both...
A non-pressurized Single can have to go to giving position reports over the high terrain out West and will pass through areas where line of site doesn't cut it. At least that was the case through the '90's. Also, crossing segments of Canada there are areas where it is possible to be out of VHF coverage and some machines don't have HF or DLC capability.
In routes that cross Pakistan, India, Myanmar and Bangladesh and parts of Viet Nam there are areas where one may be incommunicado due to frequency congestion, crappy ATC equipment (you have to hear it to believe it) likely left over from the British Empire and failure of the host nation to ensure true English Proficiency. The People's Republic of Covid is especially bad in that regard and they frequently call aircraft in Chinese and then become even less friendly when the non-Mandrin speaker doesn't answer!
Africa is a communication nightmare in general with a few exceptions. The reality of passing through is that since most of the relatively small and disorganized Corruptocracies of that Continent lack sophisticated and/or functional Surface to Air ordinance and the average turbine aircraft will be out of their airspace before they could scramble the Mig (if it is even airworthy that day) the lack of communication isn't an issue in fact so long as you're hauling azz and up high. You make your calls and try to contact the agency listed, then broadcast position reports in the blind and press on...
Listed in the enroute charts are airways that have an ATA procedure to broadcast your position periodically on the Air to Air frequency because communications with the controlling agency are so unreliable.
CPDLC is a wonderful invention-it is now possible to cross the widest part of the Pacific and only make three radio calls to verify HF frequencies! On the other hand it is dependent on both aircraft equipment and satellites, which while very good these days, neither are 100% reliable and one may find one's self going old school and trying to contact someone through Stockholm Radio...