No I haven't, I literally learned about this 30 minutes before starting the blog post. I don't think it's an unreasonable assumption that your service provider will not 40x your bill with a week's notice!
I'm not defending Slack here, but allowing this to hit the wall and then raising a stink online does everyone a disservice.
Edit: by "you", I mean "the organisation of Skyfall". It's already pretty clear from the number of people chiming in on behalf of the company that this problem has been handed out piecemeal.
From the top comment, if Hack Club was told to ignore it and not pay, I don't feel they are to blame.