Using Minecraft 1.15.2
Hi I want to make splash potion effect around furnace, that have just smelted my Item. As far as I know, there is no such event like finished smelting.

 

You should probably make a capability that attaches to furnaces, then check if your item is in the output slot of the furnace. The capability stores the previous size of the output stack so you don't generate your effects every tick, only when the stack size of your item increases. To run things every tick, you'll want to subscribe to a `WorldTickEvent` and iterate through the `World.tickableTileEntities`.