Not necessarily.
Aircraft visible on Flightradar24 (within ADS-B coverage)
When ADS-B was initially launched, it was primarily used in commercial passenger aircraft with 100+ passengers. An increasing number of aircraft including smaller aircraft types, are getting ADS-B transponders but, until ADS-B becomes mandatory it's up to the aircraft producer and owner to decide if an ADS-B transponder should be installed or not.
Common aircraft models that usually have an ADS-B transponder and are visible on Flightradar24 (within ADS-B coverage):
- All Airbus models (A300, A310, A318, A319, A320, A321, A330, A340, A350, A380)
- Antonov An-148 and An-158
- ATR 72-600 (most new deliveries)
- BAe ATP
- BAe Avro RJ70, RJ85, RJ100
- Boeing 737, 747, 757, 767, 777, 787
- Bombardier CRJ-900 (most new deliveries)
- Bombardier CS100 and CS300
- Embraer E190 (most new deliveries)
- Fokker 70 and 100
- McDonnell Douglas DC-10 and MD-11
- Sukhoi SuperJet 100
- Some newer Ilyushin and Tupolev (for example Il-96 and TU-204)
Common aircraft models that usually do not have an ADS-B transponder and are not visible on Flightradar24 (within ADS-B coverage):
- Antonov An-124 and An-225
- ATR 42, 72 (except most new deliveries of ATR 72-600)
- Boeing 707, 717, 727, 737-200, 747-100, 747-200, 747SP
- BAe Jetstream 31 and 32
- All Bombardier CRJ models (except most new deliveries of CRJ-900)
- All Bombardier Dash models
- All CASA models
- All Dornier models
- All Embraer models (except most new deliveries of Embraer E190)
- De Havilland Canada DHC-6 Twin Otter
- Fokker 50
- McDonnell Douglas DC-9, MD-8x, MD-90
- Saab 340 and 2000
- Most helicopters
- Most older aircraft
- Most business jets
- Most military aircraft
- Most propeller aircraft
Of course there are lots of exceptions from these rules. There are some older A300, A310, A320, B737, B747, B757, B767, MD10, MD11 aircraft flying without an ADS-B transponder, which make those aircraft invisible on Flightradar24 when in areas with ADS-B coverage only. But there are also some Twin Otters, Saab 340, Saab 2000 and MD-80 aircraft with an ADS-B transponder that are visible on Flightradar24 in areas with ADS-B coverage.
Sorry little off topic here.