I had a quick look at Garmin Grafana, and they do seem to get the Training Readiness data, so it looks like the API is exposing it after all. That makes me think the Garmin documentation may simply not be fully up to date yet.
They implemented it here, including the details needed:
https://github.com/arpanghosh8453/garmin-grafana/pull/17
The same seems to be true for Sleep Respiration Rate as well — this is now also available via the API:
https://forum.intervals.icu/t/respiratory-rate-from-garmin-health-snapshot-avg-during-sleep/119477/5?u=andreas_schnederle-w
Would be awesome if Intervals could import all the values Garmin already exposes 