I'd think the progressive era would end at Wilson, neither Harding or Coolidge were Progressive, and Hoover only if you stretch the term.
On civil rights for women and minorities, Harding was far more progressive than Wilson. You'd have to limit progressive issues to only economic ones to argue he absolutely should not be considered part of the progressive era, and even then given his support for the eight-hour workday, it's hard to argue he should be excluded without there being another suitable era for him to be placed in.
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