For freeing of slaves
In the previous times, people used to have slaves and slave-girls. Human beings would be bought and sold. In this regard, the Shariah had permitted the use of Zakaat to free them. However, as in today’s world, there is no slavery, thus it is not possible practically to use Zakaat in this avenue.
As mentioned in Fatawa A’alamgiri, Vol. 1, Pg. No. 185.