I don’t have the MLA Handbook 9th edition, but the 8th edition says in
2.5.2 URLs and DOIs :
When giving a URL, copy it in full from your Web browser, but omit http:// or https://.
modern-language-association.csl just copies the URL that is given as argument. I think it would be best if the style could remove the
https:// prefixes. It is currently not possible to indicate that an URL should be output without the prefix, I think there could be a new markup to do so, perhaps something like
<text variable="URL" url-style="remove-protocol"/> ?