Here's a quick tutorial explaining how to create an effect like this one
First, you have to create new camera texture, Resources panel -> Add New -> Texture -> From Camera
let’s call it “pixel camera”
Then you just have to uncheck the box "use screen resolution", scale to the resolution you want
Now create a billboard
you need to create a material for this sprite, let’s use the default one
In the resources panel select your new material and set the texture as your pixel camera and the Filtering Mode to Nearest
Your effect is done now!