def METHOD_NAME(db):
    """Drop subcategories if they are in the following:
    * ``unspecified``
    * ``(unspecified)``
    * ``''`` (empty string)
    * ``None``
    Parameters
    ----------
    db : list
        A list of datasets, each containing exchanges.
    Returns
    ----------
    list
        A modified list of datasets with unspecified subcategories removed.
    Examples
    ----------
    >>> db = [{"categories": ["A", "unspecified"]},
                {"exchanges": [{"categories": ["B", ""]}]},
                {"categories": ["C", None]}]
    >>> new_db = drop_unspecified_subcategories(db)
    >>> new_db
    [{"categories": ["A"]}, {"exchanges": [{"categories": ["B"]}]}, {"categories": ["C"]}]
    """
    UNSPECIFIED = {"unspecified", "(unspecified)", "", None}
    for ds in db:
        if ds.get("categories"):
            while ds["categories"] and ds["categories"][-1] in UNSPECIFIED:
                ds["categories"] = ds["categories"][:-1]
        for exc in ds.get("exchanges", []):
            if exc.get("categories"):
                while exc["categories"] and exc["categories"][-1] in UNSPECIFIED:
                    exc["categories"] = exc["categories"][:-1]
    return db