I don't think you can use MDI in Visual Studio anymore, the tabbed interface seems to be the only one available. Although i do remember an IDE (was it Visual Studio?) that could do both MDI and tabs at the same time.
Do you consider tabbed interfaces as MDI? I've seen some people do that but i think MDI and tabbed interfaces are too different in terms of functionality to be considered equal.
MDI and tabbed interfaces aren't mutually exclusive but if the master window does not embed resizable child windows (like a traditional MDI interface) then it's not MDI regardless of tabs.
I think VS used to be MDI tabbed. It definitely used to be MDI. But I have to admit it's been years since I've last used it. In fact thinking about it now, it's been even longer than I realised (I switched away from Windows development around the time of XP getting released so we're probaby talking close on 15 years. It's scary how fast time flies).
Visual C++ 6 was certainly MDI, but i'm not sure about the rest. I think i have 2008 Express Edition stored somewhere but... i feel a bit lazy to install it to check it out :-P. 2010 and later was only tabbed though.
But, FWIW, yes, i agree with you that the resizable child windows is what defines the MDI and tabs are orthogonal to that.
Do you consider tabbed interfaces as MDI? I've seen some people do that but i think MDI and tabbed interfaces are too different in terms of functionality to be considered equal.