Your question links to a blog post by Paul Irish, which mentions that this is now considered an anti-pattern. (Since 2017)
// used at all?
The reasoning was that you could write
//mysite.com/some/resource and it would resolve to either
https://mysite.com/some/resource, depending on whether or not the site was called with HTTP or HTTPS.
Loading HTTP resources when your site is served over HTTPS can be problematic, as browsers usually complain about websites loading mixed content. Notably IE6 would display a big error message, asking you if you were really sure if you want to continue.
So why not redirect everything to HTTPS?
Because back in 2010, when this blog post was made, HTTPS was considered expensive. Not just because Let's Encrypt didn't offer free certificates back then, but because there used to be quite a performance difference between HTTP and HTTPS. These days, this is not the case anymore, and the overhead HTTPS causes is negligible.
What happens if I use
//? Is it dangerous?
Not really, but there is also not reason to use it. The recommendation today is to use HTTPS on everything. Really, there is no excuse anymore to offer plain HTTP aside from redirecting to HTTPS. When browsers finally see HTTPS as the default, then there will be no reason for HTTP existing at all.