You cannot rejoin the same iMessage group thread once you have been removed. This is a fundamental feature of iOS.
The reason for this is that when your number is blocked by a member, iOS treats this as a permanent boundary. If this block causes you to be removed from the group, the original iMessage group is effectively closed to you. When someone now attempts to add you back, iOS prompts you to create a new chat because:
- The original thread ID cannot accept you again.
- Apple does not allow ‘re-joining’ an iMessage group once removed.
- Even unblocking your number by the member who blocked you may not allow you to be added back to the iMessage group.
No settings, toggles, or secret iOS updates can resolve this issue.
The only known instruction is to create a new chat.