It looks like the cut runs pretty straight front-rear, but I cannot see how deep it is at the tread groove.
If it is NOT that deep, if it were me and I lived in Florida, and rode "mildly" where I'd never use that portion of the tire, I wouldn't have a problem running it, inspecting it frequently for any sign of change.
If I were riding spirited around the tail of the dragon, canyon carving, or track days, I'd immediately scrap it, period. You're not going to get that tire to pass inspection at a track day, and for good reason.
But....all that said....you life, health, and well being is worth far more than what ever you'd pay for a top of the line rear tire. If it is questionable in your mind, that probably is reason to replace it.