Gotcha! I'll see what I can do to make that an option in future jams, thanks for letting me know!
I experimented using a screen shader for a different jam last year but I found that lowering the project resolution is easier to setup and you don't have to worry about the shader messing with the UI so I've been doing this since to get a retro effect. One of the recent Godot snapshots added an option to scale the 3D scene independently of canvas layers with pixelation (you could before but not with pixelation) so I'll look to use that and add an option to disable the downscaling since that's easier to change in game than the entire project resolution.