After updating to unity 2019.3.9b
TriggerEnter not work. Trigger-collision / Trigger-terrain
CollisionEnter not work too
I have switch to 2020.1 but the same. No collision enter and trigger enter detections. Yes i have rigidbody on colliders and triggers.
“Auto Simulation” not checked after update! Now TriggerEnter work 
So this is just an assumption from reading @Lars-Steenhoff posted article and from some personal experience. I interpreted the articles “Trigger-Trigger overlap removal” to mean that triggers will no longer interact with each other. With some testing i found that if i only use a single trigger it works perfectly fine. So make sure your game-objects that interact only have a single trigger between them. Hope this helps!
TLDR: Only use 1 trigger on interacting game-objects