Consider the phrases: 'the document was signed off', 'the signed-off document' and 'the signed off document'. In the first case it is correct to have the gap because 'signed off' functions as a verb but, in the latter two cases, the words function as adjectives describing the document, so hyphens are preferable.

What is the correct way to spell sign off?

or sign·off

personal approval or authorization; endorsement.

Is signoff one word or two?

signoff / sign off – Ditto above. One word as a noun, two as a verb.