Risk and safety is on a spectrum. You’ve got a wide range of answers here, so I won’t go into depth on specifics, but the safest thing by far is to not take a phone at all. However, in practice, some people can’t do that. For example, a friend has a continuous blood glucose monitor that connects via Bluetooth. This affects the kinds of protests that they feel comfortable going to.

If you do opt to take a phone, there are things you can do to make yourself safer. Wiping the device beforehand can help, if you have a good backup. Turn off biometric authentication and have a strong password/pin. Airplane mode can help, as can a Faraday bag to blog signals (though DIY ones of those can have leak problems, I think). Also having a quick way to turn off your phone is good — there might be settings in your phone to make that easier. The reason for this is because your phone is harder to break into if it’s waking up from being turned off (i.e. before you put your pin in when booting up). However, even with precautions like this, it’s pretty damn hard to protect your device if it is confiscated and law enforcement is determined to get into it. This means that if you do take your phone, be extra mindful about doing anything that might make you of particular interest to law enforcement.

In addition to the personal risk angle, it’s also useful to consider the community risk. One of the ways that Bluetooth is used, for example, is to track the motions of crowds of people. The more people who are using their phones at a protest, the easier it is for them to be tracked and possibly controlled. This means that even if your security was perfect, you might be made vulnerable by someone being more careless with their privacy. Equally though, it means that you taking what steps you can helps make everyone a bit safer.