If I leave a group chat will I lose all of the old messages?

Hello 2v3w,

Thank you for using Apple Support Communities! I understand that you need help with leaving a group chat. I am happy to help the best I can.

When leaving a group message, your messages will still be on your device. If you delete the thread, you will no longer have access to the message in the thread.

Check out the following support article, which provides some steps to take to Leave a group text on your iPhone, iPad, or iPod touch - Apple Support

Cheers!