You can hide and open "CHANNELS" on Slack. It's a Tempermoneky script.

SlackHideChannels

// ==UserScript==
// @name         SlackChannelHide
// @namespace    http://howtelevision.co.jp/
// @version      0.1
// @description  You can hide and open "CHANNELS" on Slack.
// @author       YasuyukiTsuda
// @match        https://*.slack.com/*
// @require https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js
// @run-at document-end
// @grant unsafeWindow
// ==/UserScript==

$("#channel-list").after('<a href="#" id="open-channel-list" class="list_more">open...</a>');
$("#channel-list").after('<a href="#" id="close-channel-list" class="list_more">close</a>');

$("#channel-list").hide();
$("#close-channel-list").hide();

$("#open-channel-list").click(function(e){
    e.preventDefault();
    $("#channel-list").show("fast");
    $("#close-channel-list").show();
    $("#open-channel-list").hide();
});
$("#close-channel-list").click(function(e){
    e.preventDefault();
    $("#channel-list").hide("fast");
    $("#close-channel-list").hide();
    $("#open-channel-list").show();
});