Originally Posted by Blizzard Entertainment
New and Returning Player Seeding
The first time a player steps into ranked play, we use the knowledge we have about their performance in other game modes as a general starting point for assessing their skill. We also do this if a player hasn’t played ranked in a long time, and their MMR in other modes is now significantly different than their existing ranked MMR. Currently, players who enter ranked play for the first time, or after a long absence, can place as high as Diamond 3 after completing their placements.
We recognize that being placed highly when a player is new to ranked can lead to some less than positive experiences if they lack some of the more nuanced game knowledge necessary for success in that league. As a result, new players can no longer place higher than Platinum 5 at the end of their placement matches going forward.
Additionally, we believe the introduction of MMR and rank decay will help account for players who take long breaks from ranked play. As such, these returning players will no longer be re-seeded into placements based on their skill in other game modes.