I'm old enough to remember when Skype was really neat, and it's just another in a long, long line of grievances I'll hold against Microsoft till I die.
Seems much more likely to be due to legal reasons than for security. If the chats are not retained, they can't be found in discovery. (And given chat is even more informal than email, people probably say a lot of things they shouldn't in chat).
Back to the "code smell" of the IT department... For instance, I opened a ticket for my new laptop for something that is not common, but it can be self-selected from the menu of requests to make, so it's not like it's a one-off. After THREE WEEKS of emails and chats and calls with NINE DIFFERENT PEOPLE, I found that we... STARTED COMPLETELY OVER. It would have been nice to be able to look back over the history to name and shame, and point people back to what had already been done, since, apparently whatever ticketing system they use is completely useless.
So when even simple things take a month to do in your company, having, say, 30 days of history is not unreasonable. In fact, it's almost necessary.
It does change one's usage model.
*except I can't copy paste images from slack into office documents, this is a major hassle
