I see this question asked a lot here in the forums with little to no responses or just being told to "reference the forums where Matt said it." So I decided to start a compiled list of why did they leave. It covers those who have passed away to retirees to rage quitters (those who quit in the middle of a match) and more.
Feel free to add or correct if I am wrong.
Amber Rayne (Cocaine/Alcohol overdose)
Hollie Stevens (Breast cancer)
Nikki Viez[sic] (Not really in rage, Dana quit and blamed her so she too felt she had to go)
Devi Lynn (put her own safety at risk to break holds)
Gia DiMarco (became a constant no-show)
Yellow Kitty (she was unmasked and after that didn't appear on Kink ever again)
Retired: (from industry means porn industry)
Princess Donna (from industry)
Jesse Cox (from industry)
Alexa von Tess
Bryn Blayne (from industry)
Quit After Injury: