I almost feel like I should have put a trigger warning on this post. This hard fork proposal includes so many different things that sound contrary to what we know of Bitcoin that on linguistic points alone I think it is probably doomed. I suspect Bitcoiners really don't like the idea of expiring transactions. However, the proposal is a fun way to think about how Bitcoin works, so have a look:

An expiration limit is introduced for Unspent Transaction Outputs (UTXOs). UTXOs that exceed a maximum age threshold of MAX_UTXO_AGE (e.g., 1,048,576 blocks) will be deemed invalid for spending and pruned from the nodes' active Chainstate database. The nominal value of these expired satoshis will not be injected into immediate block rewards. Instead, they will be reintroduced into the pending emission reserve, dynamically extending the duration of halving epochs to guarantee a perpetual block subsidy without adding inflation.

Basically, if coins don't move for ~1 million blocks, they become part of the block subsidy.

Coins that don't move are added to a "recycle pool" and these are added as block subsidy to blocks in the current epoch, extending it until the recycle pool is empty, at which point a halving occurs.

So if something like this were enacted now, we'd see the current block subsidy (3.125 BTC) continue to be awarded to new blocks past block 1,050,000 until all the coins that have not moved in at least 1 million blocks have been doled out in block subsidies. Only then would the next halving occur.

The author, Ricard, claims this proposal solves the following pain points in bitcoin:

1. Long-term Security Budget: As the block subsidy approaches zero, network security will rely entirely on the fee market. If fees are insufficient, the hashrate will drop, compromising the network. Proposed alternative solutions like "tail emission" violate the immutable social contract of the 21 million BTC cap.
2. Chainstate Bloat & Unspendable Dust: Full nodes must keep the entire UTXO set in RAM to validate transactions. Millions of permanently lost bitcoins (destroyed private keys) and economically unspendable "dust" (UTXOs with a value lower than the fee required to move them) generate "zombie" UTXOs that permanently penalize node hardware requirements, threatening decentralization.
3. Permanent Capital Loss: A significant percentage of the total Bitcoin supply is permanently inaccessible. This dead capital provides no liquidity or security to the network.
4. Quantum Vulnerability: Millions of dormant early-era Bitcoins rely on outdated cryptographic standards (like P2PK, where the public key is already exposed). These act as a permanent honey-pot for future quantum computers capable of breaking ECDSA.

While it sounds good in theory, I doubt it is quite so simple as this. For example, my understanding of the code that implements Bitcoin's 21 million cap is that it is pretty simple: halve the block subsidy every 240,000 blocks. It's easy to understand and reason about and people can work with that. Adding this much more complicated structure to it could have a lot of side effects that may not be immediately apparent.

The proposal was posted to the Bitcoin Development Mailing List where you can follow the conversation if you are so interested.