Before I head into my current viewpoint on this metagame, just going to get this out of the way. The posts above, generally speaking, seem to be more geared towards getting likes and shouting into an echo chamber than actually discussing the impact Marshadow has on the meta. I recognize that many users may have already had their minds made up for a long time, but posts absolutely dripping with sarcasm and passive aggression, maybe with some surface-value facts thrown in there isn't good for much more than those likes. Everyone posting here cares about the state of the Doubles OU metagame, so keep that in mind when responding to a dissenting opinion. I'm not going to bother going through to specifically call out the problematic aspects of some posts above me though, since imo that only really derails the discussion from "Is Marshadow being removed the best way to further our metagame" to "is user X good at posting about whether Marshadow should go".
Just a warning, this probably won't be a short post, because I don't believe that this is a simple issue. I assume that the contributors above who are quite experienced in Doubles have thought the issue through extensively already, and that's why they're able to reduce it to "wow this is so stupid get rid of it duh" or "who even thinks this is broken lol learn to adapt losers". However, the reality is that Marshadow lies in a bit of a gray area. Being the subject of a resuspect, there are elements of timing and meta adaptation at play that led the council to believe it may be worth revisiting, but these changes are, in the grand scheme of the metagame, rather small; expecting a close vote to stay to turn into an obvious ban or an obvious do not ban is silly.
I apologize if this comes off as pretentious or taking everything too seriously, it's possible that I'm overcompensating, but I just want to go into everything so that anyone reading this thread who maybe wanted to get reqs for a TC vote could understand the issue they're about to help decide.
There's one concern that I want to address out of the gate. I've heard talk of people worrying that banning Marshadow would revert the tier to its setup-heavy, unstable, and unpopular form before its introduction. However, remember that Swagger was a large component to that metagame, allowing Tapu Fini + Swagger user to turn a variety of bulky physical attackers (Tyranitar, Ferrothorn, Snorlax, Zygarde) into giant threats. The time between Swagger's ban and Marshadow's suspect was only a week, so there's no way to know what a metagame without Swagger and Marshadow might look like. Regardless, Smogon tiering policy encourages users to remove any broken, uncompetitive, unhealthy, *insert buzzword* element, even if that potentially prevents a larger issue. Basically, don't leave Marshadow in the metagame for the sole reason that you disliked the meta before its release.
There are a number of ways to deal with Marshadow, among them being Intimidate, redirection, faster Pokemon, speed control, and in a pinch, resist berries. The question we're left to deal with is whether the strain that Marshadow puts on the metagame, in both the building and playing phases of Doubles, is too overwhelming for the metagame to thrive. The reason both building and playing phases come into play are the myriad of sets that Marshadow is capable of running, with Z-Move, LO Sneak, and LO HP Ice all being reasonably popular, and niche options like Feint and Sash showing some use. If Marshadow were entirely one-track, the building phase would be the main time that players have to deal with Marshadow - account for its potential presence and your team will be prepared. While it's true that no one Marshadow set breaks through all its counters, in the midst of a game the user facing Marshadow has to account for each of those possible sets and make a play that won't result in a loss to any of them.
All of this isn't just saying that Marshadow is an offensive powerhouse that must be accounted for in building; in fact, similar arguments could be made for Tapu Lele, Mega Salamence, Hoopa-U, and Kyurem-B as terrifying offensive threats that have ways of circumventing their checks while tanking hits. The main point in my opinion that makes Marshadow suspect-worthy is the immediate invalidation of a variety of sets. When attempting to build around a setup sweeper, enormous amounts of effort are required to keep Marshadow from turning hard-earned boosts against you. As a result, I've often found myself 4 or 5 Pokemon into a team, then forced to scrap it because "right, Marshadow exists, there's no way this idea will work."
For the sake of argument, let's discuss "setup + support" as an archetype. Other playstyles have similar problems with elements of the metagame, with prominent examples including rain with Ferrothorn and Tapu Fini, sun with Heatran and Landorus-T, and bulky balance with Kyurem-B. The differences here lie in the degree to which the problems can be accounted for in building. Rain teams can use Terrakion or Tapu Koko, sun teams get by with Landorus-T and Kyurem-B, and bulky balance slaps on a Mega Scizor or Mega Metagross to take on Kyurem-B. However, Marshadow doesn't work the same way. A variety of setup Pokemon, from Substitute users, to non-Speed boosting stat raisers, to slow Dragon Dancers get thrown out because of Marshadow and Spectral Thief.
I voted Do Not Ban on the first Marshadow suspect test. I stand by that vote, since at that point, I saw it performing worse than the expectation of it being entirely broken and rarely ending games on its own. With the popularity surge of Z-Move Marshadow and Stealth Rock support, it's becoming clear how few Pokemon can reliably switch in, making Marshadow's job easier. Being able to build and account for Marshadow has clearly become more difficult, but I'm not here to tell you whether that should cross your own perception of the line between really good and banworthy.
While laddering for the suspect, please take a moment to consider whether you're playing around Marshadow "too much", or whether it feels like your team has to be too focused on answering Marsh and thus sacrifices its general utility. Whether Marshadow is too stringent on the metagame's ability to handle the rest of the metagame, is, in my opinion, the central question to its brokenness.
I'm not going to end this with a tl;dr, ban/dnb marsh, because I don't think it's that simple and I don't want anyone saying "Oh talkingtree probably knows what he's talking about, I'll just follow his vote." Take this time to think for yourself and analyze the metagame in-depth, and post your thoughts here to keep the conversation going! It's obviously not required to do so, but I think the exercise of having to write out your thoughts can really clear up where you might be on the subject and the discussion generated will help our community reach a consensus.
Happy laddering everyone!