orth drops to untiered without shed tail, it's an unmon without it. it's a sign of brokenness on shed tail's part that both of the mons that get it are straight garbage outside of it but are serious problems with it. honestly, i don't know why we're even still having this discussion; shed tail's been a problem since day 1. hell, people could tell it would be a problem since the first trailer we saw it in. what is this move still doing here?
This argument is flawed. You can't say "x move/ability is broken because Orthworm is viable in OU because of it". That's literally the whole point of Pokemon: certain characteristics of each Pokémon dictate their strength relative to other Pokemon. It has the same contrived flavor of "without Drizzle, Pelipper is an un-mon. Therefore, Drizzle is broken". Brokenness and unhealthiness are determined by the amount of healthy counterplay in the metagame.
Baton Pass was unhealthy in most forms because passing boosts allowed for certain Pokémon to completely circumvent their checks & counters through no merit of its own. Swagger + Thunder Wave was broken because you could have your way past counters. Naganadel was broken because NP, typing, and speed tier allow it to dumpster 90%+ of the OU metagame.
What makes Shed Tail different from just a DryPass (Sub, then Baton Pass) is the 50% health drop. This means that most users only have one shot to pass a Sub. Cyclizar overcame yhat cost by having Regenerator. Orthworm not only doesn't have that, but it's also awful to use by itself. With its slow speed, you would either:
1) play very precisely to make sure you set up a scenario where you can ShedPass without taking too much damage
2) run it with Grimm Screens (which is mostly the reason why it's in B-)
Refer to my previous post about why I think Screens + Orth is overrated (TLDR huge opportunity cost of sacrificing an extra breaker/sweeper for Orthworm). Shed Tail isn't as warping as it seems, and Orth only fits in very specific team compositions where the downsides of Orth are mitigated. Even then, there's enough counterplay that can invalidate an Orth from ShedPassing.
EDIT: As an addendum to the Cyclizar vs. Orthworm point, I think Shed Tail's functionality is different between the two. The purpose of Cyclizar's substitute was to get threatening sweepers in safely without taking any damage whatsoever and not to have them sit behind a Sub and stat boost. This action was possible because of Cyclizar's high speed and Regenerator. Orthworm rarely gets those opportunities, and its Sub is not nearly big enough (base 70 HP a.k.a. 86 HP Substitute) to stop neutral and even some NVE moves from breaking it against the frailer recipient.