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It depends on the threat model. A local police force vacuuming up cell data looking for easy targets and ways to intimidate protestors is different than a targeted investigation by a state actor to identify everyone at all costs.

The Jan 6 insurrection is a good example of how difficult it is in a real world scenario to ID specific people in a large protest, and many of them got caught because they talked about it afterward on Facebook. If you are cell phone 2,347 on a spreadsheet of 33,422 phones and the number has no associated locational data, open source information, etc. you are way safer than bringing your regular phone, while not being an easy target and being able to communicate if you need to.