Is it unethical to not give feedback on final exams or assignments?
I think there's a distinction between automatically providing feed, and having access to feedback.

Like many people, I give little to no feedback on final exams for basically the reasons you describe: students don't have a chance to see it until the next semester, and very few of them bother to.

On the other hand, when a student does come see the exam, I'll spend a few minutes answering questions and explaining the grade. I think this is necessary, because students should have the option of asking for  mistakes or inconsistencies to be corrected, and they have no way of doing so unless they know why they were marked the way they were.

Many of the accusations of unethical behavior in the discussion on the question you linked to seem to be driven by concerns about the latter issue: the suspicion that students wouldn't have access to any feedback if they requested it, and that this was an intentional attempt to prevent students from appealing their grade.