Project A was created with a custom field $$ .
Custom field $$ needs to be replaced with a new field ##
New field ## was created.
$$ was deleted/removed from the project and is no longer in view. 
It is my understanding that the original custom field $$ text would remain in tasks and subtasks from the original creations.
When I create a new task (same project) the old custom field does NOT populate.
When I assign a person to the task, the old custom field $$ populates. 
When I delete the assignee, it disappears.
I tested creating a new task without adding an assignee, but adding a date, project, subtasks, description, and comments and the old custom field $$ does not populate.
On a different note, has there been a request: to provide an option to “allow” or “ignore” deleted custom fields to carry over from duplicated tasks? or something of the sort.