it is not possible for a remote destination host to signal to a sending host that it is ready, or not ready, for more requests, in a reliable way

readiness is not knowable by a receiver, it is a function of many variables, some of which are only knowable to a sender, one obvious example is a network fault between sender and receiver, there are many more

even the concept of "load" reported by a receiving application isn't particularly relevant, what matters is the latency (and other) properties of requests sent to that application as observed by the sender

health is fundamentally a property that is relative to each sender, not something that is objective for a given receiver