The decay demotion system is applied from Mythril tiers along with the grace point system.
Grace points are points that protect against demotion due to unactivity.
Mythril ~ Titan get 3 grace points per game (max. 10 points).
After one day of inactivity 1 point is deducted for each day of not playing.
Once your grace points reach 0, 150 RP will be deducted starting from the following day.
Rank Points (RP) will be reduced depending on how long you have not played a ranked game and you can check more information In-game - Summary.