"Save password for this website" dialog with ChromeDriver, despite numerous command line switches trying to suppress such popups

You need to add these preferences:

options.AddUserProfilePreference("credentials_enable_service", false);
options.AddUserProfilePreference("profile.password_manager_enabled", false);

So your final code will look like this:

 var options = new ChromeOptions();

 options.AddArguments("chrome.switches", "--disable-extensions --disable-extensions-file-access-check --disable-extensions-http-throttling --disable-infobars --enable-automation --start-maximized");
 options.AddUserProfilePreference("credentials_enable_service", false);
 options.AddUserProfilePreference("profile.password_manager_enabled", false);
 var driver = new ChromeDriver(options);

Here is this same solution adapted to Java, as used in my code. Adapting was non-trivial so sharing here in case other Java users read this:

        ChromeOptions chOption = new ChromeOptions();
        chOption.addArguments("--disable-extensions");
        chOption.addArguments("test-type");
        Map<String, Object> prefs = new HashMap<String, Object>();
        prefs.put("credentials_enable_service", false);
        prefs.put("profile.password_manager_enabled", false);
        chOption.setExperimentalOption("prefs", prefs);
        driver = new ChromeDriver(chOption);