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 r

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.