The vast majority of cargo planes are variants on commercial airliners and are pressurized. The other class are military heavy lifters, but they can also be us

The vast majority of cargo planes are variants on commercial airliners and are pressurized. The other class are military heavy lifters, but they can also be used for troop transport purposes, which means they have to be pressurized as well. In every commonly flown cargo airframe I'm aware of, the crew has the ability to get into the cargo area if needed, which a pressure resistant bulkhead would make very challenging (you'd have to depressurize the plane or have an airlock, and at that point, see "takes a ton of strength and weight you don't have any reason to haul"). There are a few weird, custom cargo planes that aren't pressurized (the Dreamlifter and Beluga cargo compartments aren't pressurized, though the crew compartments are), but they tend to be really weird, one-off designs where it's more trouble than it's worth to design them for pressurization - or, likely, the weird oversize fuselage would have to be far heavier to handle pressurization loads against the flat sides. If you point to a random commercial cargo transport, it's almost certain to be pressurized. In the 1960s, there may have been some unpressurized ones still flying around, but they'd be at a low enough altitude for the flight crew that it wouldn't be a problem back in the cargo compartment either.