(1) It's a plan that is being touted as the most modest proposal considered yet in Congress.
Here, the that-clause is a relative clause that modifies the antecedent 'plan', so I believe it's not a complement but an adjunct. (Correct me if I'm wrong.)
But I'm not sure whether the following that-clauses are complements or adjuncts:
(2) It's unfortunate that we meet under these circumstances.
(3) I have it on good authority that you are in charge here.
(4) It's for that reason that she is currently number one.
In (2) and (3), the that-clauses are extraposed, whereas in (3) it's the cleft clause.
(1) It's a plan that is being touted as the most modest proposal considered yet in Congress . I'd call that relative clause a modifier and not a complement nor an adjunct. (2) It's unfortunate that we meet under these circumstances .
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(1) It's a plan that is being touted as the most modest proposal considered yet in Congress.
I'd call that relative clause a modifier and not a complement nor an adjunct.
(2) It's unfortunate that we meet under these circumstances.
The extraposed that-clause is the subject.
(3) I have it on good authority that you are in charge h
Integrated (defining) relative clauses are normally modifiers, so the relative clause in (1) is neither a complement nor an adjunct, but a modifier. Adjuncts are modifiers in clause structure, while relative clauses are normally modifiers in NP structure.
In (2) and (3) the underlined elements are declarative content clauses functioning as extraposed subject and complement reapecftively