We made a change via hotfix a few months ago to grant Tank specialization characters bonus threat for each slot they had unlocked on their Heart of Azeroth. In the Visions of N’Zoth update, we have changed this slightly.
In Visions of N’Zoth, once you have unlocked the Major Slot, if you are any tanking specialization or a druid of any specialization with Bear Form active, you will just have a +50% threat modifier applied to you. Changing specializations to a non-tank specialization or leaving Bear Form will remove this threat modifier.
There is no longer any bonus threat for each unlocked slot, you just get a sizable threat bump from the start. This should help reduce some issues where tanks with different Heart of Azeroth levels and slot unlocks were having difficulties with threat between each other.
edit: This threat modifier should be multiplicative, not additive. A player currently at Heart of Azeroth level 70 with all slots unlocked should have roughly the same amount of total threat modifiers before and after the Visions of N’Zoth update hits.