As mentioned in this question Should I avoid playing ranked for a while after the season starts? I believe that MMR gets "averaged" at the end of each season, however I've not seen any official source for that belief.
The formula I've seen basically says that everyone's MMR gets set to (oldMMR+1200)/2
. In other words it moves you half way towards 1200 from where you are now.
Is my belief correct? Are there any official sources to confirm what happens?
The only thing I've been able to find is this from last year which says that they would be doing a "soft rankings reset". That may be where this comes from but I've found nothing for this year.
(oldMMR + 1200) / 2
which will result in the same answer as your formula it's just 'easier'. Apart from that, I do remember something along the line of what you formulated in your question. I don't know any source