[GNAT] This restriction ensures at compile/bind time that there are no stream objects created and no use of stream attributes. This restriction does not forbid dependences on the package Ada.Streams. So it is permissible to with Ada.Streams (or another package that does so itself) as long as no actual stream objects are created and no stream attributes are used.
Note that the use of restriction allows optimization of tagged types, since they do not need to worry about dispatching stream operations. To take maximum advantage of this space-saving optimization, any unit declaring a tagged type should be compiled with the restriction, though this is not required.