Can you unsend a message on Facebook Messenger, and will they still see it?

Yes. Press and hold the message > Remove > choose 'Unsend for everyone' (not 'Remove for you', which only hides it on your side). Unsend for everyone pulls it from both sides. The catch: it leaves a small 'you unsent a message' note in the chat, so it's not fully invisible, and if they already opened the chat or saw the notification, they may have read it first. There's no strict time limit now, but speed still matters because of that notification.

'Remove for you' vs 'Unsend for everyone' is the bit people get wrong. Remove for you just hides it from your screen, they still see it. You want Unsend for everyone.

It does leave the 'unsent a message' trace, so they know something was there, and notification previews mean they might have already read it. Fast is your only real friend here.