@ajj Hey. Not all animation apps are created equal. Pencil2D is not meant to replace or compete with Opentoonz, and since its focus is purely on the pre-production and production stages of an animation pipeline, it does not (and probably will not) have these kind of effects by default which are part os a post-production stage.
In such case it’s better to just use opentoonz for that, by turning the layer visibility on/off you can export your layers individually as image sequences and then import them as toon raster levels in OT to use the effect. At some point we will look into having a direct export to opentoonz and other animation software, but this is still undergoing planning.
If you don’t want to use OT for some reason, then the most natural choice would be to use a compositing software like After Effects, which is also used in the industry a lot. There are tons of tutorials for it too.
If you can’t afford After Effects (via subscription) or don’t want to use it, you might have luck by using Hitfilm Express or Davinci Resolve’s Fusion compositor which you can get for free (and legally).
If that’s also something you don’t want, then yes you might need to use a bitmap image editor like GIMP with some of the plugin effects.
I belive the G’MIC filter plugins have similar effects. This libary is also built into Krita, and I believe it can also affect animation frames, but you might have to try that out.
The last option is simply doing this by hand (painted), however this depends solely on the level of the artist to mimic the result purely by observation so it might be a bit tough to repeat this all the time since you’d have to do it frame by frame.