Accidently deleted message chain from one contact on iphone 12 ProMax

Go to Settings/[your name]/iCloud. Is the switch for messages ON? If it is there is no way to recover deleted messages.

If it is OFF and you have a backup that was made before the messages were deleted you can restore that backup. Doing so will overwrite everything on your phone, not just those specific messages.

You can also try asking the recipient to copy the message and send it back to you.

See→Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Hello Hoogirl,

Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you have questions about retrieving messages that were deleted on your iPhone: Use Messages in iCloud - Apple Support

"When you delete a message, photo, or conversation on one device, it’s removed from all of your devices."

As there is no way to retrieve the deleted message thread, you could reach out to your wireless carrier to see if you can have a transcript of them.

We hope this helps.

Cheers!