Slitscan - how?

You could use the displace time plugin (it ships with AE) and use a gradient as the displacement map.

https://helpx.adobe.com/after-effects/using/time-effects.html#time_displacement_effect

There's a plugin for that: https://aescripts.com/slitscan/

You could try the Echo effect or any such frame accumulation effect from third-party suites. Similarly, simple time-freezing using tiem-remapping would work. The only real caveat is the quadrizillions of layers and comps you may need since there's no specific effect in AE that would actually create the final image by fetching the individual lines and offsetting them. That, for all intents and purposes, will remain the biggest hurdle, no matter what and is no doubt why the existing script is so limited, too. In any case, to properly do it you'd have to write a whole custom plug-in, so for the time being resorting to external apps would be the best option.

Mylenium