Cities can expand by incorporating land outside their current borders. Can they reverse this? I can’t recall ever hearing of a city doing this. It may not be legal in some jurisdictions.
Note that I don’t mean disincorporating the entire city, just reducing the city’s footprint. The city must still exist after the reduction.
The District of Columbia did this in 1847:
But at the time, the District was not coextensive with the city of Washington.
Some states have land that is not incorporated. Most of the pre-colonial northeastern states have no unincorporated land. In New York, where I live, all land is incorporated into cities, towns, hamlets, or villages. However, a New York state law allows for municipal dissolution. A village can unincorporate, and dissolve its government. All of the land is then either merged into an existing municipality or new municipalities are created. The usual reason for this is financial, letting the town or county run the finances and provide the services a small place can no longer afford. All parties involved have to approve these changes via referendum.
Over the past several years, New York State has taken considerable steps to eliminate
or reduce the number of local governments — streamlining the law to make it easier
for citizens to undertake the process as well as providing financial incentives for
communities that undertake consolidations and shared services. Since 2010, the
residents of 42 villages have voted on the question of whether to dissolve their village
government. This average of 4.7 dissolution votes per year is an increase over the .79
a-year-average in the years 1972-2010.1 The growing number of villages considering
dissolution is attributable to the combined influence of declining populations, growing
property tax burdens, and the passage of the New N.Y. Government Reorganization
and Citizen Empowerment Act (herein after the Empowerment Act), effective in March
2019, which revised procedures to make it easier for citizens to place dissolution and
consolidation on the ballot. While the number of communities considering and voting
on dissolution has increased, the rate at which dissolutions have been approved by
the voters has declined. That is, 60 percent of proposed village dissolutions bought
under the provisions of the Empowerment Act have been rejected at referendum (see
Dissolving Village Government in New York State: A Symbol of a Community in Decline
or Government Modernization?)
Many other special purpose districts are incorporated. School districts are most noticeable but fire, water, sewer, street lighting, garbage collection, and park or playground districts can stretch across municipal boundaries, and are often unincorporated and revised as populations and needs change over time. New York City has its own special districts.
All local powers devolve from state governments and constitutions. Every state has a unique set of laws governing these areas and presumably can undo what a previous legislature did, subject to the provisions of the state constitution. I wouldn’t presume to understand the laws of the other 49 states (does anyone understand New York State laws?), but my guess is that all cities today can unincorporate land according to local rules.
I can’t speak for any other state but a few years ago my town voted to transfer a strip on the city limits to the town next door. If they can do that, I imagine they can transfer it to into at least a contiguous unincorporated area.