How do I hide translation vertical tab programmatically?
I think I found it. 

In core\modules\content_translatio\src\ContentTranslationHandler.php, there is an entityFormAlter() method, which adds this vertical tab. It seems that this method is executed after your code. I would try changing the weight of your module (so your code will run later).

(I found this looking for the If you made a significant change string in core; doing so, I found where the translation tab was coming from.)