This is incorrect. Hybrid was always a tax based on the power that comes from the extra utility of a hybrid.
Imagine you have a warrior. Then imagine you have a duplicate of that warrior but they can also cleanse, heal, bop, bubble, freedom, etc. Obviously, warrior 2 will need some “tax” to make sure they arent overpowered.
Having an alternative role/spec that doesnt impact your other role/specs is not “hybrid” at least for the purpose of a tax or handicap. However, if your other role or spec bleeds over into your other spec, it does count as hybrid. Compare a mage to a shadowpriest for example. If the priest has all that support and healing, should they be as powerful as a mage as a dps?
Generally the answer is no. But overtime as they lowered the hybrid tax, they also added power and utility to the non hybrid classes to help compensate in the other direction as well. So the power of hybrid became weaker, the tax was reduced.
Saying a warrior is a hybrid in vanilla wow because out of 40 raiders, 32 dps, 6 healers, 2 tanks, does having the ability to be a tank count as a hybrid for tax purposes if those tank abilities have no impact on your dps role? No, it doesnt, it shouldnt, and it generally has not.
If a ret paladin by being ret lost its ability to cleanse, heal, freedom, bubble others, etc then it wouldnt make sense to tax it. Its not a hybrid anymore. Its not a multi-role class. Being a hybrid means it has a crossover spec. The actual definition of hybrid is “a thing made by combining two different elements.” The tax and label only applies if it can actually do 2 things at once, like a hybrid car.
Having a tank spec doesnt make you a hybrid unless those tank abilities bleed over into your dps spec and make you stronger as a class in a dps spec because of it. If the two are isolated, you are a multi-role class. If a disc priest had no dps spells and a shadow priest should not bubble or heal, its not a hybrid - its a multi-role class. But as you know, a shadowpriest can heal and use many of its discipline abilities, making it a hybrid.
It mattered a lot in vanilla, but by wotlk there was a lot of homogenization so the divide was weaker and as a result the tax was less. Later on classes in general become so homogenized the differentiation is almost entirely lost.