There are possible caste systems that are not oppressive. The Vilani have one. But one of the key elements of the Zhodani was always the question of whether their psionic society was oppressing the proles with mind control and thought policing or if it was actually working to make sure everyone was happy and doing the job that they would be best suited for. They always had a stratified society. That's not intrinsically a problem.Ah, so a strong caste system are bad guys? Just because it isn't the modern western ideal?
But now they have a system that absolutely relegates non psionics to the lower ranks of society, not just out of the top political jobs. Previously, there were jobs that required one to be psionic, but if you were a talented military officer or engineer or whatever, you could go as far as that talent allowed. Now, nope. You are never going to be able to exercise your talents. So if their society is harmonious, it is not because they are using telepathy to guide people to jobs that make them happy, it is because they are making them happy with the jobs they get stuck with.
And, incidentally, it makes Zhodani society significantly less efficient. Because now your best military officers, managers, and whatever else won't be in those positions, unless they happened to ALSO be a talented psionic.
Though I suppose if you are a telepath, you can just mind control the guy with the talent to do your job for you.