This post says disappearing messages are included in the backups. You have to enable disappearing messages with a timer of less than 24 hours to ensure that you can opt out.

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joshjob42 7 days ago | parent [-]

Sure but the backup happens each day and then gets overwritten/deleted when the next days backup happens (which then deletes the disappearing messages that are expiring express the next backup). It just ensures you have access to any messages that you’re supposed to have access to according to the timers on said messages.

	▲	elvisloops 6 days ago \| parent [-]
		That's not how forward secrecy works. Ciphertext isn't "deleted" unless the key used to encrypt it is also deleted. That's the point of Signal's cutting edge protocol. This undoes all of that.