Taproot transaction-digest design receives significant security revision
Taproot developers revised the proposed BIP 341 transaction digest after security review identified a way to improve what signatures commit to.
In May 2020, developers requested review of a significant change to the BIP 341 signature-hash design. The transaction digest is the data that a signature commits to, so mistakes at this layer can create subtle security or composability problems.
The revision strengthened the proposal before activation and illustrates why Taproot's development timeline cannot be reduced to a single announcement or merge. Cryptographic and transaction-design details were still being challenged and changed months after the BIPs had been assigned.