There is no way in blender 2.8 to write hair_key[x].co to hair particle system.
Simple blend file with one particle hair and script:
bpy.context.active_object.particle_systems.active.particles.hair_keys[-1].co = Vector((0,0,3))
Works ok in blender 2.79. I tried to write hair_key.co to depsgraph evaluated object, but it would not work too.