"responsive": true makes a skin responsive, but does not add the meta tag, I am interested to know that.
Out of interest what do you think this option is doing?
> I followed this approach earlier and found that Google Search Console marked the site with "Mobile Usability issue: Viewport not set".
If this is not working for you, I suspect you are running an outdated version of MediaWiki. What version of MediaWiki are you running on your pages with the Google search console?
> For this reason, I mentioned the meta tag not asked to know how to make a skin responsive! Please help me to find the solution to this.
My answer is correct, I've left some pointers to code on the main branch.
Skin's have a method called isResponsive. This reads directly from the skin option you define in skin.json:
This skin option adds a viewport meta tag to the HTML. It doesn't do anything else at time of writing: