Will I lose all user data when leaving Beta programme if running a stable build?
You won't lose your data. You'll only be sent a fresh build of Android that wipes your device if you're currently running beta software when you unenroll. 

I enrolled in the beta program on my Nexus 6P to get the finished version of 7.0 Nougat when it came out, installed it, and then later unenrolled while running the finished build of 7.0. My data was, as expected, not wiped.

tl;dr: Your data is safe, so long as you are running a finished release build (and not beta software).