I don't know what you were reading, but whatever it was it was all wrong. Dehaze IS in LR Classic, but you're not running Classic, instead you are running the perpetual license version of LR6....and dehaze is ONLY available in the subscription version of Lightroom (currently Classic).
So, have you got the wrong version installed, i.e. do you have an Adobe subscription?
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Dehaze is not a feature of any version of LR 6
If you have been using it, it must have been through either the LR trial, or you had subscribed to LR CC 2015.
I had a subscription but cancelled it.
That's why you don't have dehaze any more.