A type having a Steel does not mean spinning with Glimm is impossible by any means; it's the same principle of needing to remove Ghosts before clicking Rapid Spin in OU. Of these Steel types you mentioned, most of them do not want to switch into a potential Earth Power from Glimm, which can create mindgames and give you opportunities to spin even against teams with Steels. Poison also has a lot of good options for removing Steels, including Sneasler, Iron Moth, and Salazzle.
Completely disagree, basically nothing on Psychic can break through Muk-A besides Gallade and maybe Hoopa-U, both of which can be fairly outmaneuvered defensively or offensively. Slowking-G can also act as a check to Psychic spam. I think this matchup is pretty neutral honestly, but even if it was negative, it wouldn't be especially important given how fringe Psychic is in the current meta due to its many flaws.
The combination of Pex, Amoon, and Muk-A wall all of Water's breakers (unless you run into something niche like Zen Headbutt Urshifu-R, but even then you can live one hit with Amoon or Pex and status it). Ice I will concede you have a hard time against Bax on Ice, but I wouldn't say it's unwinnable
Usually if Tusk is using Headlong, it's Choice locked and you can exploit this with Air Balloon Glimm. Amoon also is not 2HKO'd by unboosted Headlong, and can Spore/Giga in return. I do think non-Choice offensive Tusk does extremely well against Poison, but that's the only mon on Fighting that really threatens the type.
All I said was that Salazzle is a good option, I didn't say it was mandatory and of course other picks may be preferred depending on playstyle. I think frankly you should explore team structures that drop either Sneasler or Muk-A, those two mons obviously do an incredible amount for Poison and dropping one of them comes with significant draw backs, but I think it's reasonable to drop Sneasler for something like Salazzle or Iron Moth and Muk-A for Slowking-G, as they have similar roles in teams while having their own unique strengths. That being said I do generally agree that most of the time you should use the 5 mons in A and S tier, and generally Poison suffers from not having enough teamslots to be able to defensively answer everything as I said in my original post.
Salazzle readily exploits sub set-up mons by running Encore > Protect. Moth has a different role to Salazzle; Salazzle is a utility stallbreaker, while Iron Moth is a breaker/cleaner (although they are both run to deal with Steels).
These match-ups are valuable enough to make it B tier for me. I would also say it's neutral into Ghost.
Then why did you mention Bug, Psychic, and Ice in your analysis?
Sub CM Mane does not beat Muk-A. Sub BU Urshifu-R does not beat Pex or Amoon.
"Destroy" is too far, I agree they're unfavourable matchups, but they are winnable. Poison's inherent strengths of having both a bunch of fat Regen mons as well as good breakers does allow for some counterplay against problem mons.
SV Mono players when they see a team that isn't HO hazard stack
Overall, I do agree with some of you what you say here, but a lot of the arguments you raise aren't very convincing imo. I think you're too focused on Poison's weaknesses, while somewhat ignoring its obvious strengths.
I'm also going to chip in on this point.
Outside of what, Gholdengo and rest Hoodra? Not a single steel wants to switch into Glimmora for fear of Toxic. And if the counter argument of "Toxic debris Glimmora" comes up, I'm going to have to disagree as I'm basically it from what I understand for Debris users on Poison and that is only because
A. I'm lazy and can't be bothered to run Toxic Spikes otherwise.
B. Everyone else is running Corrosion that my Debris is not as expected (or would be if I wasn't always debris)
And when they do come in and get hit with a Toxic, some of Poison's worse matchups are suddenly given much bigger holes, Gambit Dark (Sneasler can play wilder since it doesn't have to worry about the Gambit Pao 1-2 Punch), Corviknight Flying (Stealth rocks are now staying up and AMuk can cause more problems with Knock off support) Iron treads Ground (Amoonguss can now Giga drain much more safetly) all removed thanks to them needing to switch in to stop mortal spin.
I don't know why I decided to post at 3 am but here I am. What I'm trying to say and I didn't express very well is that poison suffers from 6 pokemon syndrome very badly. Forgoing any pokemon means you will loose a bunch of MU. Yes, water does indeed lose pretty hard to amoong and pex but I don't think that having 3 good MUs against top types is enough. It's a bit like grass in gen 8. It does pretty well against top types like electric and water and is considered a great anti-meta type but that doesn't dismiss the fact that overall, grass is hot garbage. I think poison falls into a similar boat. It's a great anti-meta type but that is not that good overall. For example, if you run iron moth in the last slot, you're going to lose to tran, if you are foregoing sneasler in favor of something else, gambit is going to completely screw you over. Poison teams basically have to choose what types to do better in at team preview.
Addressing some of the points you brought up.
1.You said psychic is not beating poison without gallade and hoopa-U but here is the thing, almost every team is going run hoopa-U, most teams are going to run gallade, and a lot of teams will run both. The problem is that gallade is hard to play against because more and more people are running it non choiced. Either agility/sword dance or just 4 attacks. This is a huge problem as the only pokemon that outspeed are glim and sneasler. If gallade is agility, you have to use sash glim or fake out normal gem unburden sneasler. Then there is hoopa-U. Since psychic has so many special attackers, most hoopa-U in psychic will be physical. That is very bad because hoopa-U has knock off which could prove to be devastating if you plan on using a-muk's AV. I think this MU is not completely one-sided but I will say it's very bad. With correct maneuvering, a-muk + sneasler can break through.
2. Toxic Debris Glim: basically every pokemon on your team can set up t-spikes, why use toxic debris? I don't understand why that was even relevant to the discussion as I think both me and TheRealBigC knew that while we were arguing. Yes toxic is very bad unless you run rest hoodra or dengo but if you look at the steels I mentioned, most of them don't rly care about getting toxiced. You can make a case for foretress, heatran, corv, and maybe klefki but those are defo gonna be running offensive sets. This comes back to a problem of the gen 9 meta. As pengair mentioned, most people are stuck with HO brains. There is a reason for that. Defensive teams are very difficult to pull off and I praise poison for being able to do so. For example, take steel. Steel was famous for stacking spikes in gen 8 and now it's running stuff like scarf/air balloon offensive heatran and offensive BU taunt corv. Speaking of spikes...
3. Glimmora does indeed threaten a lot of steels with earth power + toxic. However, with the rise of air balloon on a lot of steel + the prevalence of offensive sets, the problem doesn't lie with spinning anymore, it lies with glim's longevity. The reason you could maneuver around spiin block with ghosts in previous gens was because most ghosts are very frail and pursuit was a thing. Steel are famous for being very hard to KO and even if you manage to KO the steel. Your glimmora will likely be relatively low at that point. Giving the opponent the time to set the hazards back up. The essence of poison is to make a team comp that is very hard to kill and can wear down the opponent until they faint or until you have something like sneasler to clean up. Therefore, it's crucial for the spinner to have that longevity. However, glim has the stats of an offensive pokemon. Even if you invest in bulk, glim's longevity will eventually get the team to crupple. You can say I have a skill issue, but the majority of games I lost with poison are due to glim lacking the bulk to keep hazards down.
4. I said "Both water and fairy have plenty of ways to beat poison like sub CM mane on fairy and Sub BU urshifu, neither of which are particularly niche sets." Please excuse this, I don't know what kind of drug I was on.
5. I also said "This is not to add for the fact the MOST of top types destroy it: dark, flying, steel, etc." I would say most types don't destroy poison but the important ones certainly do. Prankster sab with encore and taunt is a huge problem on dark and the presence of SD boots chien with gambit ready to sweep is very unfavorable, possibly 80-20 MU. Steel is a curb stomp unless you can get dengo out early on. Flying has dragonite an offensive lando which are bakc by corv, making life very hard unless you spam the daylight out of corrosion toxic. Ghost is not even for the sole fact that a lot of zoro-h are running knock off, making it very risky to freely bring in A-muk and dengo will always be a problem esp with air balloon. I would say ghost vs poison is more along the lines of 60-40 rather than 50-50.
6. As for great tusk, if head long run sets are CB, you can indeed abuse it with air balloon glim but one CC and you are screwed. I would argue that it depends on the skill of the player but fighting MU is defo not easy for poison. It might be favorable for poison but it more like a 60-40 situation.
TLDR: Poison occupies a very important role as it keeps 3 top types in check but loses somewhat badly to the rest. I think its role is very important because this meta, like you said, lacks defensive team styles. However, if we look at the type from an overall standpoint, I cannot put it in B-tier, I can't justify it.