I believe that sites such as FlightRadar24 voluntarily allow aircraft owners to have details of their aircraft movements suppressed. In other words, while their ADS-B transmissions are captured by network feeders' receivers as normal, they don't get displayed on the website.
It may be that an increasing number of corporate aircraft owners are taking advantage of this facility. There is certainly no question of bizjets, helicopters, etc, suddenly stopping transmitting ADS-B where they did previously.