The decay demotion system is applied from Diamond tiers along with the grace point system.
Grace points are points that protect against demotion due to inactivity.
Upon being promoted to Diamond ~ Immortal, players get 10 grace points.
These tiers get 1 grace point per game (max. 15 points).
After a day of inactivity 1 point is deducted for each day of not playing.
Once your grace points reach 0, 100 RP will be deducted starting from the following day.
You can check more information In-game - Profile - Summary.
Note: Tiers that don't have grace points will not show this information.
*The Demotion (Grace Point) System might be subject to change.