I have ACF PRO and I am using it in a new website. But when I were the develop of this website I found a strange behavior to delete the fields. They really doesn’t remove from the database (wp_options) so, when I debug the general option array I look the fields yet.
So if this is the correct behavior, I would ask for a option to remove all old fields. Otherwise, if this is a bug, please fix it!! Because it is very annoying.
This is not a bug, it’s the way ACF has always worked. There has never been anything in ACF that will remove old data if you delete a field. There have been many discussions about this, so I won’t go into it here