This is the infinitive replacement of the Gerundivum.
Don't mistake the Gerundivum for the Gerundium — gerund. German doesn't have the latter. English doesn't have the former.
The Gerundivum is the third participle German has. As the other participles, it can be used like an adjective. It tells what has to be done or what cannot be done. Compare
der gerettete Plan — the plan that we have saved
der nicht gerettete Plan — the plan that we have not saved
That's the Partizip II as an adjective.
der rettende Plan — the plan that saves/saved (us)
der nicht rettende Plan — the plan that does/did not save (us)
That's the Partizip I as an adjective.
der zu rettende Plan — the plan that has to be saved (by us)
der nicht zu rettende Plan — the plan that cannot be saved (by us)
That's the Gerundivum as an adjective.
So that's the general idea of the Gerundivum. But adjectives also have a predicative use. That's with copula phrases:
der schwierige Plan — the difficult plan
That's the attributive use.
Der Plan ist schwierig. — The plan is difficult.
That's the predicative use. Let's use the participles predicatively:
Der Plan ist gerettet. — The plan is saved.
This works just the same in German and in English. That Partizip II is used as an adjective. This isn't Perfekt but Zustandspassiv. The plan is both difficult and saved.
Der Plan ist rettend. — The plan is saving. (as in life-saving)
This also works just the same in German and in English. This isn't present progressive tense but it tells another property of the plan. Just as before. Now the plan is told to be difficult, saved, and (life-)saving.
And finally, let's try that with the Gerundivum.
Der Plan ist zu rettend.
Alarm siren goes off.
Nope. It doesn't work that way. In German, we say
Der Plan ist zu retten. — The plan has the property of being salvageable, or it has the property of being required to save.
Without the -d that tells apart the infinitive from the Partizip I in German.
In my view, this is purely because it sounds better.
But that became a grammar rule.
Ist das noch zu retten?
Das ist noch zu retten.
Zu retten ist das noch?
Noch ist das zu retten.
Those are perfectly fine and both formal and "street". Native speakers talk that way all the time. Most common:
Bist du noch zu retten?
In stark contrast,
Kann das noch gerettet werden?
is grammatical and fine as well but it doesn't get you street credibility. You are clearly an Ausländer.
Oh, and as an addition, there are more copula verbs in German, so variants as
Dieser Plan bleibt zu retten. — This plan still has to be saved.
Gilt dieser Plan als zu retten? — Is this plan considered to be possible to save?
are also fine and common.