Words can't be defined arbitrarily, so I am going to examine your definition first.

First, I am not sure what exactly counts as "mainstream", and why is it even important. What you describe seems like a relationship between a meme and a culture, whether large or small. So you could have "anti-memes of antimemes" as Isnasene describes [LW(p) · GW(p)]. Or you could have a polarized society with two approximately equally large cultures, each of them having their own "anti-memes". Or a small minority, such as cult, that strongly ignores the surrounding culture.

What did you mean by "mainstream knowledge"? It is something most people sincerely believe, or just something they profess [LW · GW]? They may react differently. Sincerely believing people may listen to arguments when they have proper form; but you can't convince a person whose "belief" is simply an expression of belonging to a team.

A symbiotic war half-meme encourages you attack its parity inverse as "wrong". The meme in a meme-antimeme pair nudges you to dismiss its antimeme as "unimportant" or invisibly ignore it altogether.

I am thinking now about "culture wars" where attacking other people's opinion as wrong has gradually changed into "no-platforming". I wonder whether there is a spectrum where sufficiently "no-platformed" opinions change into "unimportant" when the side defending them is completely defeated.

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Also, I am afraid that the actual usage of the word "anti-meme" would be to defend ideas from valid criticism. ("You only disagree with me because this is an anti-meme that threatens your ego!")

The example of Lisp is a good one here: we have a decades long holy war where one side shouts "Lisp is superior (and so am I by recognizing this fact)!", the other side goes "where are the libraries? where are the tools? where are solutions to problems X, Y, and Z?", but the former side goes "la la la, I can't hear you over the sound of how Lisp is superior!". Then suddenly someone with a good object-oriented background fixes the usual problems with Lisp, creating Clojure, and -- lo and behold! -- suddenly the mainstream is happy with the result.

That is, focusing too much on how your idea is an "anti-meme" makes you blind to its actual flaws.

Antimemes are a culture-specific phenomenon. Different cultures have different antimemes.

Because cultures are nested within one-another, it's interesting to posit that anti-memes can have their own anti-memes. For instance ethically-motivated vegetarianism is an anti-meme for (most) meat-eaters but wild animal suffering is an anti-meme for (most) ethically-motivated vegetarians.

Also note that the anti-meme of an anti-meme tends not to be a meme. This is a matter of dynamics. Since the meme culture is the default, a culture bonded to an anti-meme may only exist when the meme culture has not developed a way to dissolve the anti-meme. Thus, anti-memes for cultures bonded to anti-memes must be viewed as useless from the perspective of the meme-culture. Otherwise, the meme-culture would just use the anti-anti-meme to dissolve the anti-meme.

Wild animal suffering is a good example of this. Even though people periodically bring up wild animal suffering caused by plant farming as a talking point against ethical vegetarianism, actually taking wild animal suffering seriously would be far more corrosive to the meme-culture than ethical vegetarianism (the anti-meme culture) would be.

I also think some anti-memes might also be culture-generic. For instance, utilitarianism ideology looks a lot like the anti-meme for pro-social behavior. Even if utilitarianism is discussed relatively frequently (and periodically does get attacked as wrong), it checks all the boxes in practice:

Learning it threatens the egos and identities of adherants to the mainstream of a culture[1] [LW · GW].

Utiliarianism, roughly speaking, equates saving the life of someone next door with saving the life of someone far away (which can easily be achieved relatively cheaply). This radically re-orients how moral virtue (ie egos and identities) would be assigned.

Learning the meme renders mainstream knowledge in the field unimportant by broadening the problem space of a knowledge domain, usually by increasing the dimensionality.

Utilitarianism dramatically reduces the moral importance of being involved in your local community by broadening the problem of morality to people far away who need way more help. Moral circle expansion (in the sense of considering animals more seriously as moral patients) also does this and even renders local communities unimportant depending on their complicity in factory farming and how much you care.

Mainstream wisdom considers detailed knowledge of the antimeme irrelevant, unimportant or low priority. Mainstream culture may just ignore the antimeme altogether instead.

Definitely true of factory farming. Pretty true of global poverty.

The typical response to encountering a regular meme is to assign a truth value to it via rationality.

This seems...iffy.

That's not an antimeme. An antimeme is a piece of information that, by nature, prevents itself from spreading, like an uncomfortable truth. 

The opposite of a concept conceived by a generation should not be defined by a different generation that clearly doesn't fully understand it.

Here are some base problems I have against this whole "Antimeme" thing, as I can see that there are multiple counter definitions:

1. Meme A that takes up the same category of mindspace as Meme B. Contrast this with symbiotic memes
2. Memes that obscures itself against observation (the SCP fiction definition)
3. A Memetic hazard, either culture-specific or universal

Currently is it just #1 but I hope that there are better ways of ruling this.

An antimeme is a meme with the following three characteristics: Learning it threatens the egos and identities of adherants to the mainstream of a culture

Antimemes are often a culture-specific phenomenon

If antimemes are culture-specific , they can be subculture specific.

In your previous version, it was carefully explained that LISP isn't that great. Yet you are still enthusing about it. Consider the possibility that you are the one who is clinging to beliefs for reasons of ego. Maybe "LISP" isn't that great" is an antimeme relative to the LISP subculture.