I added this to handle the case when the owner component unmounts before a request has resolved.

Nit: isn't it the same as

pollIntervalIdRef.current = setTimeout(
  sendRequestRef.current!,
  pollIntervalMsRef.current
);

Can you explain why you created a ref here? I would have used useCallback for this purpose. Was it necessary to add the ref?

I see that sendRequestRef is also a ref. I think useCallback is better for this purpose. Maybe I'm missing something?

I think switching to useCallback is an interesting idea and could yield optimization benefits by returning a memoized reference to sendRequest. It would look something like this, though I'm not sure about the implications of mutating a ref you're dependent upon (any risk of infinite loop?):

  const scheduleRequest = useCallback(
    () => {
      // Clear current interval
      if (pollIntervalIdRef.current) {
        clearTimeout(pollIntervalIdRef.current);
      }

      // Set new interval
      if (pollIntervalMsRef.current && isMounted.current) {
        pollIntervalIdRef.current = setTimeout(
          () => sendRequestRef.current!(),
          pollIntervalMsRef.current
        );
      }
    },
    [pollIntervalIdRef.current, pollIntervalMsRef.current, isMounted.current, pollIntervalIdRef.current, sendRequestRef.current],
  );

However, the original NP-ready code uses refs and I find them easier to reason about than the useCallback example above, so I think I'll leave optimization to another day.

Yeah, it would imply a bigger refactor (e.g. why do we have a ref for pollInterval?), but I agree we can leave that to another day. 👍