Depending on the protest leaving behind phones isn't a hard rule imo. Think about the people going and what the protest is about. Is there something that will actually happen where the police will put enough effort and resources to track you through your phone but not use cameras around the city to track you?

I think there is a very small amount of stuff in that tiny area. If they're willing to track people down through phones, they'll most likely also be willing to pull camera footage from everywhere in the city and in that case, having no phone won't protect you. Now you're getting into tracking your movements throughout the entire city + machine learning tracking your walking movements, body shape, etc. in which case it doesn't sound like you're prepared to defeat those methods

If you're not in a city though I guess there might not be enough cameras and ditching your phone is a good idea

But otherwise, phone is good to coordinate with the people you're planning with and most importantly, filming the cops in case they decide to do a brutality

Plus you could always just turn off your phone if you really need to