I’ve gotten two thumbs up and no objections to San Francisco, Denver, Broomfield, and Nantucket being tagged admin_level=6
, so I’ve verified those are currently that way.
If we were to lump all 87 of the special cases of tagging cities under this thread, then that’s four down, 83 more to go 
Although I believe we’ve tackled what are probably the most contentious ones.
Here’s the breakdown of the 87:
44 CCCs
- 4 single-entity CCCs (San Francisco, Denver, Broomfield, Nantucket)
- 1 Honolulu CCC
- 6 Alaskan "City and Borough of X" CCCs
- 33 Dual-entity CCCs (including 2 in Alaska)
41 independent cities
- 38 in Virginia
- 1 St. Louis, Missouri
- 1 Baltimore, Maryland
- 1 Carson City, Nevada
2 notable exceptions
- 1 Washington, D.C.
- 1 New York City
I initially thought we should move to the independent cities for something probably easier to agree on, but since we have some momentum on the single-entity CCCs, then perhaps it’s best to look at the following similar single-entity CCCs:
- City and County of Honolulu (Hawaii)
- Municipality of Anchorage (Alaska)
- City and Borough of Juneau (Alaska)
- City and Borough of Sitka (Alaska)
- City and Borough of Skagway (Alaska)
- City and Borough of Wrangell (Alaska)
- City and Borough of Yakutat (Alaska)
Note that the Municipality of Anchorage does not follow the “City and Borough of X” format, but as @stevea points out from Wikipedia, “Though its legal name is the Municipality of Anchorage, it is considered a consolidated city-borough under state law.”
Also note this is separate from the discussion on the Unorganized Borough and the censes areas.
If there’s a consensus to treat the 7 single-entity CCCs above the same as the other 4 single-entity CCCs we’ve reached a consensus on, then I’ll update Wikidata, OSM tagging, and the wiki talk table to match what was done with San Francisco, et al.
I would also like to add a note to each of those explaining they are single-entity CCCs and should be tagged admin_level=6
with border_type=county;city
(or border_type=county;town
in the case of Nantucket).
Any thumbs up for that?
And if it becomes too conversational, we can pause on these 7 CCCs, do something easier like the independent cities, and revisit.