Can you actually unsend an iMessage, and will the other person still see it?

Yes, on iOS 16 and later you can Undo Send within 2 minutes: press and hold the message > Undo Send. Two catches: it only works if the OTHER person is also on iOS 16+ (older iPhone or any Android and it stays put), and it leaves a small 'You unsent a message' note on both sides, so it is not fully invisible. You can also Edit a message within 15 minutes.

The 2-minute window is strict and both people need iOS 16+. On an older iPhone it does nothing on their end, they still see it. That catches a lot of people out.

Edit is the underrated one. If it is a typo rather than a whole wrong message, long-press > Edit within 15 minutes instead of unsending.

It does show 'unsent a message', so if they had notifications on they may have seen the preview already. Speed is everything.