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://.
but 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 http:// and 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"/> ?
Would a new value for @form make sense here? (Obviously, the problem is that URLs are currently rendered with text, so adding a new attribute like @url-style or a new value for @form will potentially cause headaches for other variables. E.g. what is <text variable="title url-style=“no-protocol”/> supposed to mean?)