A few days ago I've used https://github.com/nthitz/discord-slack-import to import the slack channels to Discord. It's not perfect (doesn't use threads) but it's one of the better tools I've found.

If somebody knows a more recent/feature rich tool, I'd love to hear it!

rereading your comment: I guess you are looking for a way to export your discord history, not import your slack history into discord. sorry!