I got this to work using the ForceCreateProcessApi setting, set to true, and adding the "-private" option to the browser command line arguments.
I also had to change the registry key: HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\TabProcGrowth
TabProcGrowth is a value under Internet Explorer\Main, not a key. I created a DWord value for TabProcGrowth under Internet Explorer\Main, and set the value to 0.
FYI, the ForceCreateProcessApi is viewed as an unsupported approach.
InternetExplorerOptions ieOptions = new InternetExplorerOptions();
ieOptions.ForceCreateProcessApi = true;
ieOptions.BrowserCommandLineArguments.Insert(0,"-private");
_driver = new InternetExplorerDriver($"{Directory.GetCurrentDirectory()}", ieOptions);