There isn’t an option to leave a group chat

See the bottom area here.

Send a group message on your iPhone, iPad, or iPod touch - Apple Support

Hey there!

If you don't see the "Leave this Conversation" button, you are in a traditional group text message, not an iMessage conversation. Group texts use your wireless carrier's text messaging plan, and since iPhones cannot directly tell other iPhones they want to leave a conversation, leaving is not an option.