Is this sentence correct they have deemed waived?

That's not a sentence. It's a fragment. To know if it's correct we'd have to see the rest of the sentence; if there is no rest of the sentence, then no it's not correct, because it's a fragment.

It's an awfully awkwardly Worded fragment, while we're at it. Why not just "They have waived" whatever? Why do they have to deem it waived? It could technically be correct in a legal document, if the deemers are not the waivers (perhaps a court of law has determined that "the party of the first part" waived something, and "they" is referring to the court).