It becomes a broken as fuck wall. Once you remove the Pokemon with Fire, Rock, Flying, Ghost, Dark coverage, and Mold Breaker, its basically GG.
Choice items become significantly less viable since picking a move that isn't SE on Shedinja becomes a complete waste of PP.
Bulk Up/Calm Mind/Curse sets now require you to have SE coverage, possibly forgoing STAB entirely or resttalk.
Defensive hazard setters get fucked hard if they aren't named Skarmory.
Defensive Pokemon in general get fucked if they aren't immune/uneffected by Toxic. Porygon2 now becomes standard in Stall with Trace.