I can’t speak from my own experience using Shedinja here, because by the time I did, in a Bug-type run of Emerald, I basically knew all the moves used by major trainers’ Pokémon, and what moves wild and route trainer Pokémon were likely to know at most levels.Do we assume that players play while constantly looking stuff up (especially NPCs' movesets) on Bulbapedia? This question is especially important for tiering Shedinja, which as you may guess, is probably by far the most matchup-based Pokemon in the whole series. Shedinja may rank pretty well if the answer is yes, but if the answer is no, then it will be bottom tier in every game.
I suppose I would say ‘no’ for route trainers and wild Pokémon, because that seems excessive, and then maybe say that Shedinja should have a more detailed ‘Major Battles’ section outlining which Rival/Leader/E4 Pokémon know dangerous moves, eliminating the need to look up those movesets?
Either way, I don’t think Shedinja will be ranked very highly, because despite its ability to wall a bunch of key threats, raising it is a pain at every stage. Nincada levels up incredibly slowly to begin with, and as karxrida said, its movepool as a Shedinja is awful until it gets Shadow Ball. It’s hard to justify using it in an efficient run.