Are old illegal air transfer methods still feasible today?
Yes, it is possible to fly "under the radar".

The type of radio waves that are used by radar and similar equipment to track aircraft generally travel in straight lines. This means that, because of the curvature of the Earth, radars have blind spots at low altitudes when you are sufficiently far from the radar. See the top illustration (not to scale):



While radar coverage today is no doubt much better than it was during World War II, much airborne surveillance today (SSR, ADS, etc.) relies on cooperative targets - which means that tracking an aircraft is a two way communication; the radar sends out a request, and the aircraft replies. The pilot could easily turn off airborne surveillance equipment, thus becoming invisible to secondary radars, ADS and so on, but would still be visible on primary radars, which work by sending out a radio pulse which is reflected off the surface of an aircraft. My point is, primary radar coverage is limited, with more and more primary radars being phased out and replaced by secondary radars, so there are definitely plenty of areas where, if you fly low enough, you can avoid radar detection.