Suggestion:
Add a method to the HumanEntity interface to retrieve the amount of absorption hearts left on the human. This method would allow developers to track the amount left on the player.

Implementation:
The method can simply forward its call to EntityHuman#getAbsorptionHearts in the NMS code.

Thoughts:
I do not know how the absorption effect works on other entities like zombies, hence those entities would need to the same method too, moving the method from the HumanEntity interface to some parents of both Player and Zombies like LivingEntitiy.