Which version of Lightroom have you upgraded to?
Dehaze is included only in Creative cloud version and not the stand alone version 6.
I am using the CC version and have never had the Dehaze slider - what should I do?
Please go to Help->System Info and state the exact version number of Lightroom shown there.
Lightroom version: 6.6.1 [ 1083169 ]
Operating system: Mac OS 10
Version: 10.11 
You do not have the Creative Cloud version of Lightroom, so you will not have the dehaze feature in your Lightroom.