There are several things that should be said about your question.
It is a bad practice to use MathJax for formatting and not for Mathematics. So it is not good to use MathJax to achieve strikethrough. (MathJax also gives information which part is mathematical formula and which part is text. This information is used, for example, by screen readers, by search engines that specialize in search for mathematical formulas, etc. In general, it is better to avoid mixing between syntax and semantics, between formatting and marking type of text - in this case distinguishing between mathematics and normal text.)
Some related posts from the past:
- MathOverflow Meta: Using MathJax for formating (bold, italics)?
- Mathematics Meta: Would HTML/Unicode formatting cause troubles?
If you want to strike out some part of the text, use <s>...</s>. The disadvantage is that it skips formulas, but otherwise I think it is clear.
I would advise against using MathJax for the purposes you want to use to link it in your question, i.e., to strike through a long text. (Of course, there are still reasonable uses when this command can be used as part of mathematical formula.)
Example: <s>This is merely an example text but ... </s>
This is merely an example text but I was too lazy to copy lorem ipsum from somewhere. To include at least one formula here, let's go with König's theorem: $\sum_{i \in I}\kappa_i < \prod_{i \in I}\lambda_i.$ I also wanted to include a bit of MarkDown such as url (I linked to Wikipedia), bold or italics to see whether it works.
To address the TeXnical issue, you need to include \require{cancel} if you want to use \cancel. See also: MathJax \cancel not working (Mathematics Meta).
That is, unless the configuration for MathJax which this site uses is not change. A bit more details on this can be found here: Poll for MathJax macros that should be automatically loaded (Mathematics Meta).
Example: $$\require{cancel}\cancel a + b= \cancel a+ c$$
$$\require{cancel}\cancel a + b= \cancel a+ c$$
In the current revision of the post you linked, the \cancel command does not work for two reasons. Firstly, you cannot expect MathJax command work outside MathJax environment. (I.e., it has to be marked by dollars or some other way which denotes MathJax.)
\cancel{ \text{Does this sequence of cohomologies detrmine the geometry of $N$? Namely is it true to say that two nonisometric metrics on $N$ give two different cohomolgy sequence?}}
Even if you add dollars it will only work if also \require{cancel} is included (between dollars) somewhere in your post. You can see here that it looks quite ugly. (And for long text it goes over the page width, which is why I copied only the first sentence. I wrote: $\cancel{ \text{Does this sequence of cohomologies detrmine the geometry of N?}}$.
$\cancel{ \text{Does this sequence of cohomologies detrmine the geometry of N?}}$
This is how it looks using html tags.
Does this sequence of cohomologies detrmine the geometry of N? Namely is it true to say that two nonisometric metrics on N give two different cohomolgy sequence?
Which was achieved using: <s>Does this sequence of cohomologies detrmine the geometry of N? Namely is it true to say that two nonisometric metrics on N give two different cohomolgy sequence?</s>