As an extension of muting channels, you can make any server's channel list a little more compact by hiding muted channels. There are a couple of ways to do this:
1. Mute desired channels
You can mute the channels you want to hide, either by clicking the bell icon in the top-right corner:
... Or by opening up the server's notification settings, accessed here:
2. Click the 'Hide Muted Channels' option in the Server Settings Dropdown Menu
Once you click the Server Name (in the upper left corner), you'll see the 'Hide Muted Channels' option in the Server Settings dropdown menu.
Clicking that option will hide any muted channels away, leaving just your desired channels: