In many states it’s illegal to use flashers if you’re moving, but I think it makes sense to use them if you are moving much slower than the rest of traffic. Some people use their flashers when moving slowly in bad weather, and basically everyone is moving slowly, so it doesn’t make much sense, unless maybe it helps see the car, but the problem is, if the flashers are supposed to mean you’re stopped it can causes traffic behind to use their brakes when not necessary, which is bad. Braking when not necessary causes traffic waves and jams.
Yesterday, I was driving 70–75mph on the turnpike and suddenly traffic ahead was jammed up and braking. I pumped my brakes, not because I was so close I needed to brake hard and worried about skidding, but because I wanted the car behind me to really notice I’m braking. Luckily, he also was not driving very close behind me, and everyone was slowing down in a reasonable way. Slowing sounds too gentle. It was 70 mph down to 20 mph in a fairly sudden way.
To more directly answer your question. If traffic suddenly slows, I don’t think to put on my flashers. It’s not part of the traffic laws in any of the states I’ve lived in to do so as far as I know. However, I would say it’s not a bad idea to use them to make sure the guy behind you pays attention that something is wrong up ahead. Sudden stops I wouldn’t have time to reach over and hit my hazard lights, that’s why pumping the brakes is my go to.
I always think to use my flashers if I’m completely stopped on a road or even on the shoulder.