For normie protestors, leaving your phone home works perfectly well

As others said, you can reuse burner phones, and they can be really really cheap. You can get a 10-year-old midrange phone from an e-waste place for free or close to that, replace the battery (which tends to be much easier on old phones), and it will do everything you need just fine. I think it's the sweet-spot between convenience (e.g. navigation is really useful if the crowd carries you to part of the city you don't know, taking photos/videos, etc) and safety (even if you get caught, and are forced to unlock the phone, there's virtually nothing on it that cops can rummage through). Just make sure to pre-download maps and other resources you may need (for maps on a cheap old device I would recommend this: https://f-droid.org/en/packages/app.comaps.fdroid), but don't log in to any accounts. Unless you really need to communicate with others over the phone, keep the airplane mode on. If you're savvy enough, while replacing the battery you can also physically disconnect the antennae from the modem too for extra peace of mind.

Oh, also, I don't know about your country but in some places you can still get "anonymous" pre-paid SIMs from sellers in shady underpasses for cash. If you really must communicate with others via cellular/need mobile internet, that's also an option to put in your burner phone. But once again, avoid logging in to any accounts or calling anyone you know unless absolutely necessary.

ask yourself why do you need it?

just leave it at home, bring some cash. need the map? try to learn the route beforehand, need communication? agree to meet whoever you met at a specific time and place. you shouldn't be online or on the phone, protest, talk to strangers, chant...

I personally don't own a phone. My solution to messaging, recording, and navigation is my Thinkpad X200T running parabola GNU+Linux-libre as the os and GNU boot as the bios. It's as anti-surveillance pro-piracy pro-freedom you can get. For protests, just turning off bluetooth and wifi on it pretty much gets you off the radar. The only problem is that it's camera really sucks. Photos are doable, but videos might not. Also, for protests, physical privacy is important too, so I hope you are wearing bandana to protect you from being identified in addition to protection (helmet, knee pads, and maybe a gas mask depending on how big the protest is)

Gaps in data is data too. Phones stay home.

just don't bring your daily driver phone, leave it home. carry some secondary phone you can use for camera or navigation (offline maps) or fm radio

Do not bring any device, even your car tracks you if it has GPS, If your face is seen on any camera, your known.

Don't bring your phone? Either that or a faraday

Look, if you're going to one of these protests, what are you really worried about? These are mostly very popular, very well attended liberal demonstrations. Is it a big deal that there is digital evidence that you were in the area at that time?

If you think (or plan that) you may be involved in more than waving a sign, then just leave your phone at home. Make a plan ahead of time, and have one or more friends in on it that do the same. Look out for one another, and wear clothing that visually obscures your identity. Wear masks, cover any tattoos, don't wear distinctive clothing.

It will help stop the phone from broadcasting your location, but the danger is the private stuff on your phone getting copied if your phone is seized. Better to use a burner phone with nothing private (such as contacts) on it. Used that way you don't need multiple burners. Just keep it powered off til you reach the protest. I'd be hesitant to keep it powered (such as for mapping) on the way there, unless you don't mind GPS track potentially being retained on the phone. OTOH they will probably track you anyway, through license plate and face recognition.

BTW the cheapest place I know of to get phones with minutes is below, especially the basic flip phones that are probably better for this anyway.

https://www.qvc.com/electronics/phones/tracfone/_/N-mlt0Z1z1393y/c.html

Apple's "find my" network can find your phone when it's turned off, because the phone continues to transmit low-energy Bluetooth which other devices in that network receive and report. So if you're in a crowd with a switched-off iPhone and other people have their devices on, it's still possible for your location to be tracked. There may be other modern phones that do this too, continuing to transmit low power signals to nearby devices. If you really don't want to be tracked, you can't be sure Airplane Mode or turning the phone off will be sufficient.

1. You can probably re-use a burner phone again and again
2. What do you need a phone for? Could it be replaced with other items? (Cameras, walkie-talkies, printed maps?)

I have a small camera (a little smaller than a phone) that can do photos and videos.

The phone will be powered off before leaving the house and only turned on if absolutely needed to make a call. Hopefully that's enough for no GPS/cell tower tracking, right? Don't need to wrap it in foil?

Wrap it in aluminum foil. Faraday cage

Just remove SIM card