If that tire is inflated to your normal riding pressure, then it doesn't look like the cut even goes through the threads of the tire casing. A cut that long through the threads should be bulged out pretty well. This cut doesn't appear to be bulged out in that picture. So it is mostly just a cut in the rubber. Which is not structural for the tire. It just protects the casing and provides a grip for the road surface.

It's your decision. Do what makes you feel good. If I was doing competition type riding I'd replace it. But since I don't do competitions, I have time to let it fail. And that might be never or the next ride. It's generally not a safety issue for most riding. IMO.