I don't see any particular reason not to add it, but there are some workarounds you can use in the meantime:

• Build a cache of all site information with the site URL as the key, and then look up the api_site_parameter value (and associated information) that way.

or

• Use the fact that the domain name can be passed in as the value for site, and clean up the returned site_url to fit, i.e.:

  var api_site = network_user.site_url.substring(
      network_user.site_url.indexOf("//") + 2,
      network_user.site_url.length - 1
  );
  

I don't see any particular reason not to add it, but there are some workarounds you can use in the meantime:

• Build a cache of all site information with the site URL as the key, and then look up the api_site_parameter value (and associated information) that way.

or

• Use the fact that the domain name can be passed as a valid value for the site parameter, in all API routes that require it. EG: /2.2/answers?site=unix.stackexchange.com

Clean up the returned site_url to fit, i.e.:

    var api_site = network_user.site_url.substring(
        network_user.site_url.indexOf("//") + 2,
        network_user.site_url.length - 1
    );