Currently I have found two ways of culling / LODs in Niagara
1, you can find in emitter update, emitter state, scalability, set to Self and adjust accordingly see attached image below
2, is a way of controlling spawn rate in a linier manner in relation to the camera by using the engine provided parameter Engine.Owner.LODDistance.
As you can see from my screen grab I created a Float clamp in spawn rate this is to control any extreme numbers which could kill the engine and is a must, enter your max particle count and min, in the float section of clamp convert it to divide, A=2800000 B=Engine.Owner.LODDistance Drag it from Parameters Engine Provided, value A can be adjusted per your requirements now move away in your preview port and watch your spawn rate drop.
this was done in 4.25 and is a bit of a hack, it can also alter particle size, color and possibly alter materials with dynamic parameter to reduce shader counts on the fly, some creative maths required but it does add to the instruction count but it can be very powerful if implemented correctly.
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