I mostly agree with Michael's points about this tag, and in particular I think his comparison to a plot-hole tag is apt, because all those animation-mistakes questions he links are pretty much asking about plot holes, incredibly minor ones introduced by mistakes during the animation process.
Let's run down some of the criteria that other SE sites use to decide whether to keep a tag.
The Stack Exchange Meta guide for when to burninate a tag suggests asking yourself the following questions when deciding whether to do away with a tag:
Does it describe the contents of the questions to which it is applied? and is it unambiguous?
It kind of describes the contents of the question, but it can never fully describe them. I wouldn't say it's unambiguous. "Animation mistake" is not terribly precise. Is it a mistake if the animators drew something off-model? Is it a mistake if they didn't do it on purpose, but not a mistake if it was intentional? Is it a mistake if a character's hair is a little less floofy in one scene? Is it a mistake that they ever animated El Cazador de la Bruja? (It was definitely a mistake that I sat through that entire miserable sod of a series.) The question Why is Dragon Ball Super's animation quality so terrible? uses the tag in a completely different way than the other questions, but I can't say that either way is wrong based on the tag's name. The tag wiki unhelpfully tells us that the tag is "For questions related to mistakes in anime-series." If we do decide to keep it, we definitely should clarify what kind of questions it goes on.
Is the concept described even on-topic for the site?
Yes. Moving along.
Does the tag add any meaningful information to the post?
Kind of. Not really. The questions Michael links are all basically about plot holes, so the most important thing is knowledge of the series, not knowledge about animation. If we used this tag only for questions that required technical knowledge about the animation process, it would add information, just as tagging a question with both a series tag and japanese-language adds information. (The Dragon Ball Super question actually does use the tag this way.)
Does it mean the same thing in all common contexts?
Pretty much. Animation mistakes are mistakes in animation. The term is ambiguous (what constitutes a mistake?), but that's a definitional problem, not a contextual problem.
According to these criteria, this is not a great tag, but it's not world-endingly awful. It's mostly harmless and can safely be ignored if we choose.
Science Fiction and Fantasy has used a rather strict nine-part test to decide whether a tag is allowed to exist. Recently there's been some controversy over it, but let's give it a try, as long as we don't take the results too seriously. A tag gets +2 points for each "Yes", +1 for a "Sometimes", and -1 for a "No".
- Does it make sense to be an "expert" in proposed?
SF&F defines "expert" for their purposes like this:
For our purposes, being an "expert" in a tag means: 1) if the only thing someone knew about a question was that it was tagged proposed, that would be enough information for them to feel confident about trying to answer it, and 2) it's reasonable for someone to have a lot of knowledge about proposed as a separate subject, and not only because it's a subset of a larger topic (e.g. would someone know a lot about light-sabers on their own, [or] only because they're an expert on everything about star-wars)
It does not really make sense to be an expert in animation-mistakes. To know the answer to any of these animation-mistakes questions, you do have to be an expert in the series (other than the Dragon Ball Super one, which represents a whole different problem with this tag). -1.
Does it make sense for a question to be tagged only with proposed?
Not really. If any of those linked questions hadn't been tagged with the series and had only been tagged animation-mistakes, they would be missing vital information. -1.
Does proposed have a single, universally-unambiguous meaning?
As I argued above, the meaning of "animation mistake" is ambiguous. It made sense as a tag for the One Piece questions about arms that should have be bitten off and gloves that may not be on the correct hand, but it also makes sense for Why is Dragon Ball Super's animation quality so terrible?, which is not the same kind of mistake at all. -1.
Is proposed likely to be used correctly just based on it's name?
I'm not even sure what the correct usage is, so I'm gonna have to say "Sometimes". +1.
Are there "enough" (> 15) but not "too many" (> 10% site-wide) questions that qualify for proposed?
We could probably dig up a couple more that could have used this tag, but I'm not sure about 10 more. -1.
Are people like to use proposed to find questions to answer?
I doubt it. I have my favorite shows. I like answering questions about my favorite shows. Some of those questions might come up from an animation mistake. But I can't imagine myself ever just wanting to answer a bunch of questions that come up from animation mistakes. -1.
Are there likely some users (be objective!) who will favorite or ignore proposed?
Apparently this tag has three followers. So I guess this is a +2.
Could proposed be reasonably used to feed questions to a specialized chat room?
Maybe. Gonna say +1.
Can proposed be used to search for questions (for any reason) in a way that keyword searching cannot accomplish?
Yes. There's no obvious combination of keywords that would bring up all five animation-mistakes questions, so +2.
Very roughly speaking, tags that score > 12 are "good" tags, tags that score < 8 are "terrible" tags, others are likely good but may need some clarification/renaming/etc.
By my count, this tag scores +1, so it's pretty terrible.
Based on these criteria, I agree with getting rid of this tag. Because it's on so few questions, this won't be a lot of work, and I really don't think the tag adds much.
The Dragon Ball Super question is the only one where I think the tag adds necessary information. However, it was closed as a duplicate of Is intentionally producing a lower quality TV release unique to anime?, and that question is tagged anime-production, which describes its content even better than animation-mistakes. So I think we should get rid of animation-mistakes, tag the Dragon Ball Super question with anime-production instead, and reduce to other four questions to just the series tag.