What do you mean?
the [detachable] should not be written in the editor, or it should not be there at all?
If your system is not in void-safe mode, this sounds normal to display it with [detachable]  .. since potentially everything is detachable.

However, maybe you suggest that we display [detachable] ONLY when there is really the "detachable" keyword?

To me, it would be the expected behavior. I also noticed this today
for instance a class such as
   class FOO
   feature -- Access
   bar: STRING
   end

if I auto-complete "ba|" it will suggest bar: [detachable] STRING




So I would expect to see the [detachable]  (in non void-safe system), only when there is indeed the "detachable" keyword.
And I would also expect to see it in the feature flat formatter.