https://support.riotgames.com/hc/en-us/articles/204010760
Demotions and Decay
There are two possible ways to drop below your current division and tier: decay and demotion. Decay sets in after you don’t play for a long period of time; you will begin to lose LP until you play a match in that queue. Demotion occurs if you lose enough matches at 0 LP in a division or tier.
DECAY
Diamond, Master, Grandmaster, and Challenger are all subject to decay. In Diamond, after 28 days of inactivity in any ranked queue, you'll begin losing LP based on your current tier. Every seven days after that, you'll lose LP until you play a match in that queue. Diamond players lose 50 LP each time they decay.
Master, Grandmaster and Challenger work a bit differently:
Each game you play in Master, Grandmaster and Challenger tier is stored as a "banked game," up to a maximum of 10.
Each day, 1 banked game is removed from your reserves.
When your banked games reach 0, you'll lose 250 LP per day until you bank more games.
Wins and losses both count toward banked games; all that matters is that you play. You can track your banked games in your profile, in the Ranked tab.
If you fall at or below zero LP as a result of decay, you'll be placed in the next-lowest division. If you're already in division IV of your tier, you'll fall into the next tier down (for example, Gold IV to Silver I).
Decay doesn't occur for players in Iron, Bronze, Silver, Gold, or Platinum.
DEMOTIONS
A particularly bad losing streak may result in dropping a tier or division, though there’s no set number of games that may trigger a demotion. Losing enough games at 0 LP can drop you into the next-highest division. If you’re in division IV, it’s possible you’ll even drop a tier. After a demotion, your LP will be set to 25 or 75, depending on how many losses you had at 0 LP.
After being promoted into a new division or tier, you’ll gain the effects of a demotion ‘shield,’ which prevents you from dropping. This is to keep you from winning a series, losing a few games and then immediately dropping back to where you were. The shield lasts for 3 games after advancing a division, and 10 games after advancing a tier (3 games for Master tier).
you should be safe as plat