Perhaps this
markdown: redcarpet
redcarpet:
extensions: ["strikethrough"]
Github flavored Markdown and pygments highlighting in Jekyll
Or
echo '1. <s>Call Mom today.</s>' | kramdown
Result
<ol>
<li>
<s>Call Mom today.</s>
</li>
</ol>
Note if you are using jekyll --watch
this config change will not be picked up; you will need to restart Jekyll.