First of all, don’t listen to Cuthbert. He’s like actually delusional and does nothing but confidently spout conspiracy theories without providing any evidence. Second, I’ve experimented a bit with off season to see what effect, if any, it has on MMR, and the conclusion I’ve come to is this:
Off season competitive does have an MMR that is initialized to your final MMR of the prior season, or, if the account is fresh, then it is initialized to the ‘default’ MMR equal to around 2350 SR. This MMR, however, does not have any effect on your actual competitive MMR, and at the end of the off season it is discarded.
The Alternate Account
The first piece of evidence I have is one of my alternate accounts. I bought it at the end of season 10 and got it to level 25 the day before the off season. I didn’t touch competitive until the off season. My first off season competitive game was the same SR that it would have been if it were the first placement game - a mid gold game. After about 7 games, I was being placed in mid diamond games, indicating that the system was recognizing improvement at about the same rate as placements even in the off season and was attempting to place me in the right rank. I stopped once I hit my first master game at about game 15.
Then season 11 started and I queued up for my first placement game. If off season MMR impacted competitive MMR, then it would make sense I would be placed in a master game, for that is where my off season MMR was. But no. I was placed into the default rank of mid gold for my first match, and I had to work my way back up through the placement matches as if I hadn’t already done that on the account.
The freshly placed account
Second, I had an account which I placed on the last day of season 11. It was level 27 and 3000 SR. I screwed around on Torb and lost pretty much every game in the off season, and by the end of the off season I was in mid gold games. The next season I did placements, and it put me right back in diamond games, and it ended up placing 3300. If off season had an impact on its SR, I would have been put back in gold games and been placed in plat.
The main
And then finally I have my experience on my main account. Whenever I play in off season on my main, I usually stack with my silver-gold friends and we end up in high gold games. And usually I’m just messing around, so we lose a fair amount of games. Since I’m a mid master player, if off season MMR affected comp MMR, it would follow that the next season I should place lower. But every time I’ve placed no lower than 100 SR of where I left off, and a couple of times 100-150 higher.
So I think it’s safe to say that off season does not affect the next season.