> You transmit information by changing the content of the transmission
First off, you're using the word in the definition. You can't use "transmit" to define "transmit". A transmission is the noun variation of transmit (verb).
Second off, a transmission is *active*
Think about radio. If I am constantly producing a 440kHz signal then I'm transmitting a signal. If I'm not producing the signal, I'm not transmitting.
You are not considered to be transmitting unless you are holding down the button to send the signal.
That's how a canary works. You're constantly transmitting a signal (the canary is constantly singing) and then all of a sudden it goes quiet. You have stopped transmission.
Does this communicate? Yes. But what it communicates is ambiguous. Maybe the canary just went to sleep. Maybe it starved to death instead of getting carbon dioxide poisoning. It does not provide an unambiguous truth.
That reasonable deniability is the reason a canary works. You can claim it was taken down for other reasons, such as an accident. Those reasons have to be believable and justifiable. Mind you, a warrant canary can work like going down in one commit and up in the next, happening over a small period of time. A canary does not need to work by continuous existence or continuous absence.
Canaries also frequently work by having expirations (which is closer to how you're thinking, but still follow the same abstraction discussed above). It has to be manually updated or modified. For example I could add the canary "godelski hasn't been raided by the FBI: signed 31 oct 2025 expires 7 Nov 2025". Were that message to still exist exactly on Nov 7th (and it will because I can't edit comments outside a time window) then you can conclude that my canary expired. You can't conclude I was raided by the FBI. You should be suspicious, but you can't be positive. Maybe I just can't update comments...
This isn't to be conflated with the way we transmit information is through variation, such as high and low in binary. Technically while you're talking you make pauses and "stop talking" several times while saying a single word. But we say you're talking until you stop "transmitting" or complete. If this pause wasn't included then the dead would still speak and your annoying uncle would never shut up