When broadcasting of some TV channel is cut off (for example, as part of censorship), what do you call that action? 'TV channel is [past participle]' — fill the blank, please. Can I say, for instance, 'brought off the air'?
"Defunct" properly expresses what you are trying to, referring to something that is shut down, not in operation, and/or doesn't exist anymore. Most people would likely say "That TV channel was taken off the air" or potentially describe it like "That TV channel doesn't exist anymore" in common speech, however this phrasing is clunkier than using the word "defunct."
"That TV channel is defunct" is the most accurate and correct way I can think of phrasing the sentence.