Slack is not doing anything. It's Google chrome filling up your clipboard with the "extended" version of the url.
But when I paste it in the Slack message box it shows the ɢoogle.com version.
I haven't used slack, but I think both are doing the best practices around there: Chrome copies the punycoded URL to clipboard, Slack will decode pasted punycode-URLs into a nicer presentation.
The fact that we need application-specific security measures against this just emphasizes the problem. There are a lot of applications.