FWIW: I (@rondy) am not the creator of the content shared here, which is an excerpt from Edmond Lau's book. I simply copied and pasted it from another location and saved it as a personal note, before it gained popularity on news.ycombinator.com. Unfortunately, I cannot recall the exact origin of the original source, nor was I able to find the author's name, so I am can't provide the appropriate credits.
In Fall 2023, Slack introduced an updated client with some unpopular UI changes. The tweaks in this document can be used to revert to the old client or hide the new side bar. These will reset every time you fully close Slack but they are easy to re-apply once you get the hang of it.
To run any of these scripts, you'll need to open the Dev Tools (or Console). You can do this by typing /slackdevtools in a Slack channel or direct message. These are the same Dev Tools you'd get in Chrome if you hit F12. Along the top are tabs for Elements, Console, Sources, Network, etc. You'll want to paste these scripts into the Console tab at the > prompt.
If Slack disables this command, then you can open Dev Tools by setting a system-wide environment variable and using a keyboard shortcut to open the console. If /slackdevtools worked for you then you can skip these next two sections.