If someone unsent a message, there was probably a good reason why. The window is 2 minutes so honestly it’s either a mid-send and meant for not your eyes, there was an error and the person wants to unsend and then type the message again or it’s not relevant anymore like maybe they asked a question and then answered it themselves before you got to reply.

This is the feature working as intended, why do you want to know what was said so badly? You could always ask too if you really want to know.