I don't have an answer, but it always surprised me what people would say either in Direct Messages or in private channel. This is a corporate slack, I'm pretty sure management can access all conversations, even in DM or private channels.
You can't directly access DMs, but if you pay slack enough to export data for data compliance (I think it's the most expensive tier but it may be a lower one, but definitely not the free one), DMs are exported in that file, so you can indirectly get access to DMs that way.
Even if weren't possible now, it may be possible in the future because Slack (in practice) owns and controls that data without any oversight from you. Anything you say might be deleted after a while, or might be held indefinitely.
Well... Yeah. I didn't intend to imply that it will never be a feature (it might already be in higher tiers), and I never intended to imply anything about Slack's data retention.
I was just stating that at the moment, at my tier, I am unable to see private messages.
Your employer can always change your password and then login as you to read your private messages. It leaves behind a trace (you've gotta reset your password afterwards, they can't restore your old one), but it's a "workaround".
User ID, Username, Email, Account Creation, Account Type, Days Active, Messages Posted, Daily/Weekly Active Members, Daily/Weekly Members who Posted, % Public Messages, % Private Channel Messages, % Direct Messages.
On a per channel basis you can see members who posted, members who viewed, changes in members who posted, reactions added, members who reacted, # messages posted, total membership count.
Those are what I've found just poking around. I dont really check the analytics of our workspace so I may be missing some. It doesn't look like there is a super-easy way to look up the last time a specific member was online - but there very well could be depending on the plan. Or I could be missing where I look it up.
I don't think we have the most enterprise version of Slack, but as an admin the data available to me is # of days active of the last 30 days and # of messages sent over the last 30 days.