Supports have been seen to be taking the summoner spell, Heal, onto the Rift instead of the usual Exhaust or Ignite. This has happened in the past, but it did not become meta, so why is it such a huge thing now?
The answer revolves around both the Windspeaker's Blessing keystone, and Ardent Censer. This typically only applies to supports like: Janna, Soraka, Nami, Lulu, Taric, Karma, because they have either an additional heal or shield in their kit.
By using Windspeaker's, the support is able to grant his/her AD carry an increase in both armor and magic resistance for three seconds. A combination of both Heal and shields on an AD carry will basically make him/her harder to kill, resulting in potentially more kills for your team.
Ardent Censer also benefits from Heal because it enhances the allied champion's basic attacks who has been healed or shielded for six seconds. This is another great bonus for AD carries, especially for ones who deal a lot of damage consistently.
Not to mention, this bonus applies to every allied champion who gets healed or shielded, so if the support has a global heal, the entire team will get buffed.
It doesn't mean Exhaust/Ignite are completely out of the picture, but their stats do not benefit the AD carry as much as Heal does. Ever since Exhaust got nerfed with the removal of the reduction of the target's attack speed, Exhaust is not as good as it used to be. Ignite is only meant if the bot lane is playing aggressively and can pick up easier kills.
In the meantime, Heal is the better option for supports to pick if they want to buff their AD carry during fights.
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