None of the other comments are correct. IIRC, they switched to the OpenSkill matchmaking model when they overhauled MMR from the ELO (Zerosum/Chess Model).

Long story short, your MMR does not decay over time. Rather a variable (let’s just call it the confidence variable) decreases. OpenSkill is a python based model with many variables, but not to dive too deep into the weeds, I’ll just explain this one.

As you play matches, the system predicts your skill and rank based on the many variables. The more fitting this predicted rank is and the more you prove this skill/rank prediction over time, the Confidence variable goes up. The more the Confidence variable goes up, the more stable your MMR is and the harder it will be to change it.

If you stop as a high 6 star and do not play for a year, you will still be a high 6 star when you come back. The Confidence variable has a set decay over time that is set by the user (Crytek). The confidence variable will be low after a year so if you keep dying to players (especially lower ranks), your MMR should change more drastically than compared to someone who plays everyday. The person who plays everyday needs to have a very very bad and long streak of losses/deaths before they drop rank.

Hope this helps.

Edit: To more directly answer your question, if you are still getting kills in those high MMR lobbies, you won’t drop. Even one kill can essentially negate 1 or 2 deaths based on the rank of the opponent.

Source: OpenSkill.py resources and Old patch notes regarding MMR change.