NOTES: undo or cancel changes

Unfortunately, it is not possible to restore deleted messages in iOS 15. This was only possible in iOS 16, where a Deleted Messages folder was added to the Messages app.

The only option you have is if you have an iCloud Backup from the time when all your messages were on your iPhone. However, you should be aware that if you sync your messages to iCloud, your messages will not be part of your iCloud Backup, and they will be gone forever.

One last thing. If you have an iCloud Backup - and you do not sync your messages with iCloud, then you will have to restore your iPhone, and again restore from that iCloud Backup. Unfortunately, it is not possible to restore just the messages.

It’s probably too late now, but if you have “shake to undo” enabled that might work.

"NOTES: undo or cancel changes : I deleted a chunk of the message and I want it back."

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Recovering Lost Messages:

Did you Ever Export these? Take this as a learning experience,. knowing to create a PDF document of a messages thread; something I do all the time. It wouldn't recover then back to your Messages app, but it will be on record. See my User Tip: How to Save or Print a Complete iPhone Messages Thread - Apple Community