An aside: This post definitely changed things, without asserting that it was a change. Here, it is asserted that group size has no effect in determining win probability and hence matchmaking.
This did not use to be the case:
The other issue is how do we model the synergistic effects of players being together in a group. As you noted, they have access to voice chat. Now here’s where things get interesting. This “massive” advantage actually differs based upon the skill rating of the group members. Based upon the data we’ve seen groups of low to mid SR players don’t see that much improvement to their win %. Higher SR players do see more notable improvements, but it’s not as huge as you might think. Still, we do take this into account when we predict the win% for each team. Regardless of how the data looks, we do know there’s a perception of a large advantage for groups. That’s one of the reasons why we explicitly try to match similar sized groups together.
from Overwatch Forums
There was no admission of when this change happened. However, it was detected by Your Overwatch (Overwatch: Has Blizzard SECRETLY Fixed Ranked!? - YouTube) a month ago.