The founders comments in the hacker new thread, plus some other errant ones from hack club users.

Here is there “we are staying on slack” comment:

Hi, update here (this is Christina, Hack Club cofounder): looks like Hack Club is staying on Slack.

Thanks to all of you for the appreciation and support for Hack Club, and for listening to what we were going through. The support has been amazing. Hack Club has so many cool teenagers coding awesome projects, making friends and solving problems together, and it’s great to see so many people championing them. We are glad to stay on Slack and want to do so much more with them together going forward.

Thanks to Denise and the Slack leadership team for reaching out here on hn, and in a call directly with me and Zach today. And thank you for restoring Hack Club’s terms with improvements. We really appreciate it, and we’re glad to be able to stay on Slack.

I just want to add that it was great to get to know Mattermost and the team- and the hack club engineers were actually pretty excited to move there. It’s an amazing product and for it to be open source is awesome.

She doesn’t mention the “new rate” but it was apparently confirmed internally, and there is a comment in that above thread that lists it.