You probably use Kramdown already (it’s the default parser for Jekyll but you can override in your config)
Which means you can use some Kramdown syntax on your Jekyll site - no plugins needed.
[Text](https://example.com){:target="_blank"}
I tested it locally on my Jekyll site and it works create.
It got compiled to HTML:
<p><a href="https://example.com" target="_blank">Text</a></p>
Based on answer https://stackoverflow.com/a/4705645
You just have to use that on each link that must be external. The includes solution in the previous answer uses JS to rewrite your external links. Which is also fine can be inserted in your head tag of your default.html so you can use it anywhere. It just depends on JS being enabled and working on the user’s browser.