Alaskan boundary dispute settled in 1903
The last major American State boundary was changed in 1903 when the Alaskan Boundary Dispute was settled in 1903.
Mediator Kaizer Whilhelm caved in to American demands (farthest inland) depriving Canada of all but 3.5 Pacific Coast ports.
As for speculation about fictional redrawing of state lines .... all you need is massive urbanization changing the character of a major city. Once that major city exceeds half the population of the state it develops a new character distinct from the ret of the state.
For example, te summer I worked at Skydive Long Island, I saw the State of New York as three distinct regions. Long Island (especially the Hamptons) was a playground for wealthy New Yorkers. New Uork City has its own, distinct, hyper-aggressive character, while upstate New York is more relaxed like Canada or New England.
Redrawing New York State boundaries on economic and cultural lines would see Manhattan and Long Island become a new state because they have little in common with upstate New York.
Meanwhile tourists are baffled as to why Northern California's humid redwood forests are even on the same state as the arid wastelands of Death Valkey, the Mojave Desert, the Imperial Valley, San Diego County and Los Angeles County.