As a GSC council member I feel obliged to post here; also noting that fellow councillor Conflict has said he lacks posting rights.
As has been made evident by a few people here, it is not clear what is banworthy, broken, or "problematic" here. I'm going to frame my thoughts in terms of each Pokemon that can take advantage of trap passing or trapping and then look at the options that have been raised so far.

Smeargle mostly relies on surprise value, as usual. To be more specific, it relies on the opponent guessing the set incorrectly or respecting the more dangerous possibility, which it can do because it is used in a lead in a mode without team preview. It can pull off wonders in games that it works, but these are currently the exception rather than the norm. It's difficult to play into offensive teams, which are much more common.
The other use it can see is as a trap passer in a longer BP chain. Only a few prominent players have tried to pull this off due to the lack of anti-phazing options available, and especially because Smeargle cannot deter them with its own damage threat in 95% of cases. Umbreon is considered a much better option for this role, owing to its bulk. Smeargle does have a couple of edges over Umbreon, specifically its superior speed and access to Encore.
When considering whether to take tiering action due to Smeargle, I would say it is balanced sufficiently by its lack of stats. It is very dangerous into some matchups (against passive stall), but this requires a matchup gamble from the user. Additionally, in order to actually take advantage of trap pass, one is required to use Pokemon that can actually gain an advantage against the Pokemon Smeargle pulls off a trap against, which causes inflexibility and arguably further weakens the strategy as an archetype. Smeargle beats virtually nothing on its own, which makes this even harder to pull off.

Umbreon is where the real potential for brokenness is with regard to Trap + Baton Pass. With all of Confuse Ray, Swagger, Charm, Sand Attack, Mud Slap, and Zap Cannon at its disposal, it can realistically threaten any team. Unlike with Smeargle, its utility is not entirely absent if it cannot immediately pull its strategy off, as it is much harder to punish for making repeated attempts.
Confuse Ray and Sand Attack do not have any truly reliable answers when combined with Mean Look. Although they are not likely to prevent a singular phazing attempt, Umbreon is more than capable of pulling off additional attempts, especially if it evades status.
Even without Baton Pass, Mean Look Umbreon can be dangerous, especially Sand Attack variants through their ability to a) PP stall foes, and b) set up opportunities for dangerous teammates against phazers.
Umbreon also has moveset uncertainty as an advantage. Although Toxic is currently not too common, it is quite viable and can punish phazers and Explosion users that switch in too eagerly.
Unlike Smeargle, a trap pass Umbreon can realistically fit on almost any team, which further amplifies its strength.
If we are to take any tiering action over trapping and / or Baton Pass, in my opinion it should primarily target Umbreon's strategies.

Although it is not a trap passer, Misdreavus can pull off a similar strategy to Umbreon with Confuse Ray or Attract. It is virtually guaranteed to at least trade evenly if it pulls off a ~25% chance confusion self-hit into double Protect or re-application of Confuse Ray, and usually will pull off a KO and stay somewhat healthy.
Like Umbreon and Smeargle, part of its strength comes from moveset uncertainty. It can run Toxic or Thief sets with equal or superior viability to Perish Trap sets, and guessing wrong can be punishing for the opponent
Its weaknesses are its vulnerability to Pursuit, its lack of bulk, its vulnerability to status, its lack of utility when at low health due to its shaky recovery (reliance on Protect), and the possibility of the foe using Explosion on the turn Perish Song would KO them.
Like Smeargle, it is particularly punishing against more passive teams and can easily turn into dead weight against offensive teams. In my opinion, its flaws sufficiently balance it.
What to target with tiering action, if anything?
Here I will weigh up different options that have been raised.
First, my attitude towards tiering action, for those who don't already know, is approximately as follows:
- We are not designing our own game; we are rather playing the game as it has been given to us, with restrictions that promote expression of "player skill", but as few of those as possible.
- Banworthy elements are typically those that invoke the RNG which are not practically avoidable, have a high chance of success, significantly influence the game, and don't have a high opportunity cost.
- They could also include Pokemon that severely limit strategic choice, which can cause games to depend too much on one or two team building choices, thus inhibiting expression of player skill.
It is this tiering perspective through which I determine whether or not I think something should be banned.
I do think that Umbreon is toeing the line with its TrapPass + RNG-forcing options. At this stage, I don't think the other Pokemon are at the level where they need to be considered too much. I would like to see these strategies used more often so that I can form a stronger opinion. Currently, I would not support action in the immediate future, but there is still utility in enumerating the consequences of taking certain types of action. Therefore, here are my thoughts on each suggestion so far.
Banning trap + Baton Pass Tony's suggestion
This would gut Smeargle considerably, hurt Umbreon a lot, and do nothing to Misdreavus. Without the ability to pass, Mean Look Umbreon could PP stall, try to set up a 1v1 with Curse, or try to generate a setup opportunity for a teammate by crippling a foe and then switching. These options are all extremely susceptible to phazers.
Banning trap + Confuse Ray Siatam's suggestion, could be extended to some other RNG-forcing moves
This wouldn't touch the currently prevalent Smeargle strategies, but it would eliminate the parts of Umbreon and Misdreavus that lack reliable counterplay. I feel this is a more appropriate option to consider.
Banning Baton Pass various others' suggestion
This would turn Smeargle into an almost entirely unviable Pokemon, hurt Umbreon about as much as the Trap Pass option, not touch Misdreavus, and also hit a lot of other random Pokemon like Jolteon, Scizor, Espeon, and Vaporeon. With the possible exception of ABR's team which sets up a midgame Pass, very few Baton Pass teams have proven to be consistent or worthwhile at high levels of play.
There are a couple of advantages of banning Baton Pass over the other options involving bans, namely ruleset consistency between generations (since it looks like we are heading this way in other gens too) and simplicity. However, it seems unnecessary to fully ban the move at this stage, and I believe in managing each tier's needs individually as a priority over consistency between generations. I don't believe we are at a point where we can call Baton Pass a broken singles move as a general statement; as usual, it should only be viewed through the lens of each metagame's context.
---
I also want to draw attention to a replay from the recent GSC Invitational which displayed the strength of Umbreon:
Loser Bracket Round 5, TC v Star (Game 2, Star up 1-0)
For context, in the
previous game in the series, Star had brought the same six Pokemon with the same lead Pokemon (although with a different set). This may have influenced TC's decision to leave his Snorlax in against Umbreon. While he had Lovely Kiss to potentially get past it despite the trap, this game showed how Umbreon can exert high levels of pressure, take significant advantage of the opponent's potential assumptions about its set, and fit effectively onto a variety of teams. However, it's worth noting that Star would probably have struggled this game if TC had respected the possibility of Mean Look more.
And if you only read this sentence, my general position is that I would prefer to wait longer and see before taking tiering action.