Good to know you leave references until the end. I will not be able to get a reference if I leave. My company's policy does not allow employees to be a reference for anyone who is leaving.
This isn't uncommon, and as I understand it follows directly from the typical HR policy of only confirming the essentials, such as title, dates of employment and reason for leaving. Companies care more about possible litigation than they do about helping you with your next job.
However, in my experience it's haphazardly enforced. If you're on good terms with a colleague or manager, it's my understanding (IANAL) that they can still provide an informal or personal reference. In practice, for most prospective employers this is just as good as a formal reference (since most companies these days have the aforementioned policy anyways).
My company requires 3 references. Hiring manager does calls those just before offer is made and there is intent. I don't like calling references unnecessarily. There is a form which mostly is about how candidate interacted with co-workers and management and general effectiveness at job. Medical stuff does not come up generally. Personal references do not work well here in general for technical hires unless it's an intern and they worked together on school project or something.
My guess would be that many of the references you call do in fact work for companies with a no-reference policy. This has at least been the case at all of my employers (large and small places alike, some household names), save a 5 person startup.
In my experience, although near-ubiquitous a no-reference policy mostly seems to mean "if anyone calls the company line or shows up on prem, we redirect them to HR who then tells them nothing". It doesn't mean they go out of their way to stop individual employees from giving positive references on their own time (to wit I've never seen any employer actually make any effort whatsoever to disseminate their no-reference policy to employees, it's just a CYA measure they adopt if communication happens through channels they're directly accountable for)
However, in my experience it's haphazardly enforced. If you're on good terms with a colleague or manager, it's my understanding (IANAL) that they can still provide an informal or personal reference. In practice, for most prospective employers this is just as good as a formal reference (since most companies these days have the aforementioned policy anyways).