Afaik, there isn't a default stock API. Possibly the best approach we have found is to have a schedule process (every 5 mins) that would run a search on the item record type & qualifies items that have had a change in the "LastQuantityAvailableChange" field value since the last execution of the flow (IOW, in the last 5 mins). This would return all items w/ a change in the Quantity Available regardless of the trigger (SOs, POs, TOs etc)