Slack can't just willy-nilly hand over US government data to the public. There's a process that needs to be followed for FOIA requests as some classes of information are just not public. In fact, most of the data on a government Slack server would probably fall under those FOIA exemptions.

Not when I worked in the US govt. We were told that everything we did in our Slack was FOIA-able. FWIW.

FOIA requests go to the information owner, not to anyone hosting it. It's not even specific to IT. Paper notes/documents are also covered.