Why no Member entity in the API v4 Explorer
Appears that API4 is not the default API, we are still on API3. I was misled by the fact that API Explorer V4 links now appear on CIVICRM menu into believing that it was now default, however on my install there is still a 

/civicrm/api

path for the API V3 explorer which allows management of a much larger set of entities, including Membership and many more.