Since you mentioned it in your post. What benefit is there to Slack if the internet archive can load the chat page? Is there any value to Google to indexing the slack app? There's nothing of value there to anything other than logged in users so why would they build for anything with the access level similar to an unregistered user?

On the other end, their home page should be accessible and some I can't check right now, I can only guess that it is.