It looks like Slack’s page about this shut down is only available when logged in: https://my.slack.com/account/gateways (linked to from https://get.slack.help/hc/en-us/articles/201727913-Connect-t...).

At least as of this comment, Slack’s blog and status site don’t say anything. It seems like this warrants at least a post on the blog, but maybe that’s coming later today.