I don't understand this question. I don't think it's very common in real-world applications to deliberately trade confidentiality for extra availability, though negligently and subtextually making that trade is endemic to our industry.
I just mean availability in the sense of client support across time as protocol versions increase, like TLS version negotiation and how it enables the gradual rather than immediate dropping of server support for clients on older versions of a spec. Or for example, allowing 2048 bit keys for a time to allow migration to stronger keys.
Without negotiation, you'd have to stop serving clients that haven't upgraded at the cutover time.
