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This is some 1960s level of optimism lol. While I applaud the authors enthusiasm, this will break as soon as a warship shows up at your free city, and your society (made up of people who avoid governments as much as possible) turns out to not be willing to fight for yours. This doesn't have to be a warship - I would wager that at first major obstacle many would defect.

The closest historical analog seem to be Jewish communities of medieval europe, perhaps up to 1900s. They could sometimes get different treatment, and had some structures, although never as centralised. Some countries even wanted them for their knowledge/skillset and connections (?) which lines up with digital nomad's attributes.