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.
Best Answer
I'm not 100% sure what the exact formula is, but you are correct. There is sort of a soft reset between seasons but I'm not completely sure how strongly it is. When you are at lower/mid ELOs you don't notice the push too much, but it's very noticeable at high ELOs (which is completely supported by the Math). Last season I ended up around mid to upper Diamond. When this season started, I began in Platinum 1 after doing extremely well (7/10 I believe) in my placement matches.
TL;DR; Yes, but I'm not 100% sure on the formula but it's likely something close to that.