I had a custom field setup for a project but later I realised it was not necessary for the tasks, so I deleted it, however, the entry box is still showing, just greyed out.
This is not a bug but by design
We had a lot of users who didn’t want to loss the information stored in the Custom Field but no longer wanted to have the Custom Field in the new tasks.
That is why when you remove a Custom Field from a project the existing tasks would have that Custom Field data but the Custom Field would stay disabled. While any new task created in the project would not have that Custom Field.
OK, that makes sense, perhaps it should still be possible to delete it from a task if its really no longer needed, but I can accept the logic behind this