Is the expression "true" superfluous and/or not a predicate?

pp. 17-28

in: Paul Weingartner, Basic questions on truth, Berlin, Springer, 2000

Abstract

When one omits the predicate within a declarative sentence consisting of subject and predicate the meaning of the linguistic expression changes radically and one does not have a sentence any more. Therefore: If "true" in ""?" is true" ("? is true")¹ is a superfluous addition in the way that there is not a single case, where ""?" is true" ("? is true") means more than "?", then "true" cannot be considered a predicate. As Frege states, "true" in ""?" is true" ("? is true") is a superfluous addition:"One might be tempted to regard the relation of the thought to the True not as that of sense to reference, but rather as that of subject to predicate. One can, indeed, say: "The thought, that 5 is a prime number, is true." But closer examination shows that nothing more has been said than in the simple sentence "5 is a prime number". … It follows that the relation of the thought to the True may not be compared with that of subject to predicate."²