client/lib/plugins/store.js
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;
			} );
		} );
	}
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;
			} );
		} );
	}
Array of sites without a particular plugin.