code: function( sites, pluginSlug ) {
  		const installedOnSites = this.getSites( sites, pluginSlug ) || [];
  		return sites.filter( function( site ) {
  			if ( ! site.visible ) {
  				return false;
  			}
  			if ( site.jetpack && site.isSecondaryNetworkSite ) {
  				return false;
  			}
  
  			return installedOnSites.every( function( installedOnSite ) {
  				return installedOnSite.slug !== site.slug;
  			} );
  		} );
  	}
docs: Array of sites without a particular plugin.