Not every airplane does that. Commercial planes mostly do, but its not mandated under all circumstances as far as I know. (Speaking for my region only)
All commercial planes have it at this point. And most part 91 airplanes (General Aviation) have it at this point. Without ADSB you cannot fly in or near class A, B, or C airspace and must stay below 10,000 feet. Which largely makes having an airplane useless for most people.
There are still airplanes flying that have no electrical system but even then some of them have retrofitted a system.
I was answering with the purposes of flight-taxis in mind. I doubt that they will fly in high altitude and deem it quite likely that they might want to pass uncontrolled airspace at some point.
And if you're flying in uncontrolled airspace you just can't count on the presence of it. In fact, missing transponders are an occasional source of accidents in these spaces. There are lots of gliding enthusiasts that aren't equipped.
P.S.: Commercial planes (esp. big ones) should have hardly any contact points with flying taxis.