What exactly is the purpose of not testing classes that are ready?
Focus feedback? That's not going to happen, someone who plays hunter, priest, paladin, will not give you feedback on shaman, druid, mage if those are the classes available. A die-hard arms warrior will not give you feedback on fury. You virtually just stop any feedback at all on focused channels. If there is any datamineable content for a class present, then you'd run into severe speculation cluttering all forums - harming the actual feedback.
In the past devs usually cycled through their assigned classes, work on feedback (maybe), and make changes. Then swap to another class and do something with the feedback. The thing is, why delay classes that are ready to generate feedback for this? If changes are planned, the classes are not ready. If no changes are planned until feedback, the classes are ready.
There is only one reason for not all classes being present: They're not ready. That means we have at most 4 months for class and talent testing. Unless they changed something, we also know that ~1 month before pre-patch goes live class changes stopped (Ion even talked about that in this interview:
https://www.mmo-champion.com/content...le-for-Azeroth). So while we have 5 months until the end of the year, you can effectively substract 2 weeks of pre-patch and 1 month of no more changes. That's 3.5 months of testing and feedback. Releasing classes over the course of a month is, well, pretty bad.