I'm using LWRP in Unity 2019.1.12f1. The UI is in Screen Space - Camera. The shader used for the particles is Lightweight Render Pipeline/Particles/Unlit (Transparent, Premultiply).
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Camera Stacks are now part of Unity's Universal Render Pipeline (URP). You can add an overlay camera over your main scene camera to render the particles - https://bdts.com.au/tips-and-resources/unity-ui-particles-in-urp-using-overly-cameras-and-camera-stacks.html