This is still a concern in 2025. If your aircraft systems break, or if you don't want to be identified, there are surprisingly few ways of identifying you nonetheless.

It surprises many people to learn that we do not have full radar coverage of the continental United States, much less the oceans. Outside of the ADIZ (Air Defense Identification Zone), military bases, large airports, etc., planes are more or less tracked voluntarily by systems like ADS-B.

From the excellent Computers Are Bad newsletter, https://computer.rip/2023-02-14-something-up-there-pt-I.html :

""" It is a common misconception that the FAA, NORAD, or someone has complete information on aircraft in the skies. In reality, this is far from true. Primary radar is inherently limited in range and sensitivity, and the JSS is a compromise aimed mostly at providing safety of commercial air routes and surveillance off the coasts. Air traffic control and air defense radar is blind to small aircraft in many areas and even large aircraft in some portions of the US and Canada, and that's without any consideration of low-radar-profile or "stealth" technology. With limited exceptions such as the Air Defense Identification Zones off the coasts and the Washington DC region, neither NORAD nor the FAA expect to be able to identify aircraft in the air. Aircraft operating under visual flight rules routinely do so without filing any type of flight plan, and air traffic controllers outside of airport approach areas ignore these radar contacts unless asked to do otherwise.

There are incidents and accidents, hints and allegations, that suggest that this concern is not merely theoretical. In late 2017, air traffic controllers tracked an object on radar in northern California and southern Oregon. Multiple commercial air crews, asked to keep an eye out, saw the object and described it as, well, an airplane. It was flying at a speed and altitude consistent with a jetliner and made no strange maneuvers. It was really all very ordinary except that no one had any idea who or what it was. The inability to identify this airplane spooked air traffic controllers who engaged the military. Eventually fighter jets were dispatched from Portland, but by the time they were in the air controllers had lost radar contact with the object. The fighter pilots made an effort to locate the object, but unsurprisingly considering the limited range of the target acquisition radar onboard fighters, they were unsuccessful. One interpretation of this event is that everyone involved was either crazy or mistaken. Perhaps it had been swamp gas all along. Another interpretation is that someone flew a good sized jet aircraft into, over, and out of the United States without being identified or intercepted. Reporting around the incident suggests that the military both took it seriously and does not want to talk about it. """