Basically a starting point of hiding channels and blocking users in Slack

better_slack.js

document.addEventListener("DOMContentLoaded", function() {
  let hiddenChannels = ['general']
  let blockedUsers = ['Slackbot']

  let customCss = `
    .p-channel_sidebar__static_list div[role="listitem"]:empty { display: none; }
  `

  hiddenChannels.concat(blockedUsers).forEach(it => {
    customCss += `a[aria-label~="${it}"] { display: none; }\n`
  })

  let s = document.createElement('style');
  s.type = 'text/css';
  s.innerHTML = customCss;
  document.head.appendChild(s);

  let remover = function () {
    setTimeout(function () {
      hiddenChannels.forEach(it => {
        document.querySelectorAll(`a[aria-label~="${it}"]`).forEach(it => it.remove());
      })
      blockedUsers.forEach(it => {
        document.querySelectorAll(`a[aria-label~="${it}"]`).forEach(it => it.parentNode.remove());
      })
      remover();
    }, 250);
  }
  remover();
});

Yes, I know. You might think: if you need this, something is wrong with your organisation. And maybe that's true. I just don't care and want more control about what I'm exposing myself to on Slack.

How to install:
Append Gist contents to this file: /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js

How it works:
It just adds some CSS and removes DOM elements. That means, you have to mute the channel you want to hide (usually, that's general). Regarding users: It will not hide notifications correctly. Not sure what to do about that yet.