sorry wrong window

You can delete messages if you catch it fast enough. up-arrow, delete content, save and Slack will ask about deleting. Or use the context menu, and delete it directly. And don't worry, we've all done it 😉

You don't even have to be fast. You can click the three dots in the top right corner of your message, and delete it at any time. 🙂

That used to be the case, but slack changed the default time limit to a couple of hours and now the admin changed it to 24h, see #meta.