I don't know, because typically they don't broadcast the fact that they've been dumped, in your words—that they have been excluded from the program.
Typically, most programs claim that they have a very high rate—sometimes 100%—of success in protecting their witnesses while they are witnesses. Of course, one can become suspicious of that and say, well, what happens once you stop protecting them, and are you not stopping the protection of those who are high risk just to keep your statistics going?
You will hear that, for instance, about the federal marshal program in the U.S., where they have the reputation that they don't lose witnesses and all their witnesses are safe. But critics of that program have said yes, but those who are at risk you have excluded, so of course you never lose any, because they get killed afterwards. I don't know whether that's true, but that's the criticism.