If you wrote it on your company's slack space, it is not really private to the company. Everytime I have a "private" conversation in Slack I have the understanding that the information could be shared. Even now there is a share button not for the whole conversation but yes for individual messages
It's potentially open to the public as well, given the right legal action. The Enron email dump revealed secrets that presumably ended several marriages.
Well the Enron dump was in the early 2000s, a lot of that stuff (irrelevant to the litigation) would get sealed these days. There were things like social security #s and bank accounts in there.