No. Decaying in League of Legends has no impact on your MMR. If you decay from your current rank to a lower one your MMR will stay higher than your actual rank. For example, if a Diamond 1 player decays to Platinum 1, he will continue playing with D1 teammates.

Does normal MMR affect ranked MMR league?

Does ranked MMR affect normal games, TFT, or flex MMR? The answer is no, each game mode has its separate MMR calculator and is not interlinked with each other.

What is the maximum MMR difference?

Year 6 ranked change: max MMR difference between players in a squad changed from 1000 to 700.