The FAA allows certain aircraft to have their ADS-B data blocked from internet websites. However, there's a big loophole — this only applies to websites that obtain their data directly from the FAA; it does not apply to people who have their own ADS-B receiver hardware. This has resulted in a flourishing number of private ADS-B receivers and ADS-B aggregating sites.
A few aircraft don't transmit their location. If there are enough ADS-B receivers capable of MLAT within range, then they can use the time-of-flight information[?] to calculate the aircraft's position.