According to the documentation,

is_year_end Logical indicating if last day of year (defined by frequency)

With the help of other users in the community, I have understood that "defined by frequency" means that, for example, if the series contains only business days, the accessor .is_year_end is not going test if each date in the series is 12/31, but it will test if each date in the series is the last business day of the year.