I'd be curious to know this as well, I can say for sure that the WotLK Raids can no longer be entered by 10's at least. My understanding is it was the change to Raid groups that "fixed" it when WoD came out.
So 6.0.3. I tried ICC a bunch of times after I saw this, couldn't get in. I think 30s can still get in but not sure. 20 was a no go. I tried with xp on or off too.