The iMessage editing and unsend features are being toned down. You can only change a message for five minutes within the available 15 minutes, and the unsend window is reduced from 15 minutes to two. Previously, you only saw a notice that it had been edited, but now you can see the history.
The decision was not explained. Apple has been asked if it can give a reason. The previous approach has the potential for abuse. Someone could change a message to say you agreed to something. They could either harass you or disappear before you can preserve the evidence.
It's possible to prevent edits and unsending by disabling iMessage. Those features aren't available in the text messaging service. Apple's moves won't completely prevent harassment, but they may encourage you to leave iMessage on without fear.