def METHOD_NAME(table: Table, year_threshold: int, metric: str) -> Table:
    """Create projection and historical versions of a metric.
    Parameters
    ----------
    table : Table
        Input table.
    year_threshold : int
        Year after which the data is considered a projection.
    metric : str
        Name of the metric to split.
    description_base : str
        Description of the metric.
    Returns
    -------
    Table
        Modified table. Should have two new columns: `metric_historical` and `metric_projection`.
    """
    # Get mask
    mask = table["year"] < year_threshold
    # Add historical metric
    table = _create_metric_version_from_mask(
        table=table,
        metric=metric,
        mask=mask,
        metric_suffix="historical",
        title_suffix="(historical estimates)",
        display_name_suffix="",
        description=_modify_variable_description_historical(table[metric].metadata.description),
    )
    # Add projection metric
    table = _create_metric_version_from_mask(
        table=table,
        metric=metric,
        mask=-mask,
        metric_suffix="projection",
        title_suffix="(future projections)",
        display_name_suffix="(future projections)",
        description=_modify_variable_description_projection(table[metric].metadata.description),
    )
    return table