The proposed design includes the addition of an nExpiryTime field in the taproot annex. This field indicates that the transaction must not exist in a block at or above the specified height.

To implement this design, the mempool should organize transactions into buckets based on their nExpiryTime. When a block is reached, all buckets with an nExpiryTime less than that block height should be dropped.

Additionally, the proposal suggests adding an OP_CHECKEXPIRYTIMEVERIFY opcode, which has similar behavior to the OP_EXPIRE opcode proposed by Peter Todd. This new opcode would check if the nExpiryTime field exists and has a value equal to or less than the top of the stack.

The main difference between the two proposals is that the email's author suggests adding an explicit field for the nExpiryTime value in the taproot annex, whereas Peter Todd's proposal suggests an implicit field within the CTransaction.

The author hopes that by making the use of transaction buckets explicit through the nExpiryTime field, the design will be better implemented by non-Bitcoin-core implementations. The principle of "explicit is better than implicit" is cited as the rationale behind this choice.

Overall, the email presents an alternative design for the taproot annex, outlining the addition of an nExpiryTime field, organizing transactions using buckets, and proposing a new opcode for checking expiry times.