This is reported by the official Twiiter developers of the game. Recall that previously Valve announced a new game search system, which was due to start in a couple of weeks after the introduction of the main client patch Dueling Fates.
The first season of our new matchmaking system will begin on November 22nd.
— DOTA 2 (@DOTA2) November 16, 2017
Valve revised the system of rating matchmaking for people playing rating and non-rating games, replacing the rating with a semi-annual seasonal system. These regular seasons ensure that the MMR is an accurate indicator that will lead to an improvement in the quality of the games for everyone.
During each season, all players will receive medals in their profile based on peak skill for the entire season. There are seven different medals, and in each medal there are five stars of progress to make it to the next medal. Your current medal, as well as the medal of the previous season will be displayed to all players before each match.
At the beginning of each season, all players will calibrate the MMR, relying on the rating of last season. Your current preseason MMR will be recorded on your profile, and players will be able to track their current seasonal MMR.