Regardless of which supports/items are strong/weak, we're pretty much slaves to the rotating meta.
This in part due to the fact that the concept of a Support is pretty generic. (You support your team how? doing what?) The best supports are going to synergize with their teamcomp. And the ideal teamcomp is dictated by the meta.
Taking this meta for example, with the increased prevalence of Tanks. The ability to poking them down with a mage is less likely. More so with a support mage. And as a result, the team becomes more reliant on the ADC for shredding the tank, increasing the pertinence that your ADC is protected/fed. Making your best support options either pick supports who can get your ADC fed, or an Enchanter/Warden who can bolster them for days.