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It turns out that custom ruby plugins don't work on GitHub pages because of security concerns.

I'm trying to add a plugin (this one) to the _plugins folder of my Jekyll project, but when I deploy it to GitHub it is ignored.

Question: Is there a way to workaround this? Has anyone found a solution?

Note: Obviously I can generate html files locally and commit them to my repository. But that's not what I want.

Without a plugin

A reading time script does not require a plugin. I have created a collection of scripts that can be added without using a plugin. You can find them here. A reading time script is one of them.

Here you find the code:

% capture words %
 minus: 180 
% endcapture %
% unless words contains '-' %
 divided_by: 180 
% endunless %

Note that this code contains only Liquid and no Ruby. Therefore it can be used in your layout or in an include (without a plugin).

Optimizing the script

Suppose you have something like this:

<p>lorem ipsum</p>
<p>lorem ipsum</p>
<code>lorem ipsum</code>
<p>lorem ipsum</p>
<code>lorem ipsum</code>
<p>lorem ipsum</p>

Then you could remove the above code blocks like this:

% assign preprocessed_content=post.content 
 replace: '</p>', '__/p__' %
% assign truncated_content=preprocessed_content 
% assign cleaned_content=truncated_content 
% assign cleaned_content=cleaned_content 

Ofcourse this can be extended to support more tags.

Using the plugin anyway

If you REALLY want to use a plugin you can let your local machine or CloudCannon build your site and push the result to Github Pages. See also: https://learn.cloudcannon.com/jekyll/using-jekyll-plugins/

You need an alternative to those plugins.

As detailed in "Building a Series List with Hugo Shortcodes":

Ruby plugin execution is disabled altogether on Github pages:

Plugins on GitHub Pages GitHub Pages is powered by Jekyll.
However, all Pages sites are generated using the -safe option to disable custom plugins for security reasons. Unfortunately, this means your plugins won’t work if you’re deploying to GitHub Pages.

You can still use GitHub Pages to publish your site, but you’ll need to convert the site locally and push the generated static files to your GitHub repository instead of the Jekyll source files.

I understand you mention:

Obviously I can generate html files locally and commit them to my repository. But that's not what I want.

Still, a static website generator (compatible with GitHub pages) like Hugo is to be considered in your case.

R.J Lorimer adds:

Hugo has the concept of Shortcodes, which are much like “Liquid Tags” in Jekyll.

Also like Jekyll, you can create custom shortcode tags.

However, the major difference is that in Hugo you can create them without resorting to actually writing Go code - see Create Your Own Shortcodes.

Because Hugo uses Go Templates for rendering the pages, shortcodes can use any and all Go template functions inside of them, as well as a whole list of custom Hugo functions added to help. This makes it arguably more powerful than a liquid-template solution, but still in a template file that can be easily updated on the fly.

Plus, Hugo does support MathJax, as seen in this article.

Update Nov. 2018: with Hugo 0.52, this tweet confirms (referencing this thread):

The inline shortcode is similar to the way Jekyll allows you to use Liquid tags within Markdown