I made a firefox/chrome extension [1] based on Kevin Hafy's script [2].

Firefox Addon: https://addons.mozilla.org/en-US/firefox/addon/disable-slack... Chrome Extn: Pending Review

Per Sean0-'s comment, it looks like this might not be necessary though.

Edited: formatting

[1]: https://github.com/pocc/no-wysiwyg [2]: https://github.com/kfahy/slack-disable-wysiwyg-bookmarklet