Is this extension secure?
I'm glad you asked.
Do you want me to answer that? I'd say YES, it is secure because I don't want to steal your Slack data. But right after that, I have to tell you that it's exactly what I would say if I wanted to steal your data. I also have to say that once you install the extension, I can steal your Slack data. But I won't. Pinky promise.
Once you install the extension, you will see a notice that you need to accept for the extension to do anything.
When you install extensions that modify sites you use (like Refined) they can steal your data and break the site you're using. What's worse is that even if you inspect their code, authors can update them without your knowledge. I actually wrote an article about it with suggestions as to what you should do if you really want to run this in a secure way.
Now, if you decide to trust me... there's one more thing: this extension can break Slack for you. I go into some ideas to troubleshoot (and how to report bugs) in the bugs page.