Can the pilots just put anything in the flight field? It would appear that they are catching on…
It is something that’s manually set into the transponder, yes.
In Europe it is widely used for ATC and almost everything will be sending either an operator code + flight number or a tail number.
In the US it’s pretty random what you get.
I believe Europe is much stricter on that (make use of the data input). The US ATC system is still bailing wire and bubble-gum. As long as you have a Mode-A and a Mode-C all the rest is just gravy.
I have seen some dodgy settings, like “Schlong”, as well as the usual “For Sale” and “test123”