What Is a ‘Deep Reorg’ and Why Is It More concerning for Network Security?

A deep reorg is a chain reorganization that affects blocks that were mined many blocks ago (e.g. 100+ blocks).

It is highly concerning because it reverses transactions that the network and users had long considered final, fundamentally breaking the economic trust in the blockchain's immutability. A successful deep reorg is a clear indication of a successful, sustained 51% attack or a major protocol vulnerability.