After some testing, when I add the reservation, I get an update callback, but the reservation duration/"until datetime" doesn't seem to be part of the payload, and when the reservation expires, I receive no update callback.
Is there any way to get this reservation information from a callback or query it for a particular sku? In a previous post I made, it was established that you cannot query draft orders or draft order line items by sku or product ID, so that's out.
It seems it may be an internal only field. Please consider exposing this for use. It is impossible to keep our inventory management system fully in sync with Shopify without it.