Zcash is preparing to retire its oldest privacy technology, a move that could leave millions of dollars in cryptocurrency permanently inaccessible to their owners—even if they still hold the passwords and private keys to their wallets.

According to ZecStats data updated Thursday, approximately 12,224 ZEC remain in Sprout, the network's original privacy pool. At recent market prices, those holdings amount to roughly $15 million to $16 million that could become effectively frozen once the network activates its NU7 upgrade.

"NU7 (block 3,543,405) makes v4 transactions invalid, so what remains in Sprout is unspendable: frozen, not burned," ZecStats says in its information card for Sprout.

The upgrade is scheduled to activate at block 3,543,405, expected Thursday evening in the United States. The precise timing depends on how quickly the blockchain produces new blocks rather than a predetermined clock time. As of publication, the latest block was 3,543,027. The remaining Sprout balance is tracked on ZecStats' public dashboard, so any last-minute withdrawals ahead of activation should be visible there before block 3,543,405 arrives.

Why Zcash Is Freezing Millions in Crypto

Unlike Bitcoin, whose transactions are generally visible to anyone examining the blockchain, Zcash lets users conceal transaction amounts and the addresses involved. It does so through zero-knowledge proofs, cryptographic tools that can verify a transaction is valid without exposing its private details.

Sprout introduced that technology when Zcash launched in 2016, making it one of the earliest production deployments of zero-knowledge privacy on a major blockchain. The team subsequently rolled out improved privacy systems, including Sapling, Orchard, and Ironwood. Sprout has been closed to new deposits for years, but existing holders could still withdraw their funds.

NU7 changes that by eliminating version-4 transactions, the only format capable of moving coins out of Sprout. According to the network's technical specification, transactions using that format will become invalid after activation.

The decision addresses a problem familiar to anyone maintaining legacy software: supporting obsolete technology adds complexity and potential security vulnerabilities. Sprout's original privacy system demanded substantial computing resources, and its outdated transaction-verification mechanisms create risks that developers want to eliminate. The episode also illustrates a trade-off that ordinary software deprecations don't face: disabling a legacy spend path leaves any funds still on it with no route out, even for owners who hold the correct keys.

Developers have not technically destroyed the coins. They will continue to count toward Zcash's circulating issuance calculations, and a future network upgrade could theoretically restore access—though that is not guaranteed, and the team has shown no interest in doing so. The specification explicitly warns that spending those funds may never be re-enabled.

What Happens to Zcash Holders?

Legacy holders are the people most exposed to the change. Anyone whose ZEC remains in Sprout must move it through a compatible withdrawal mechanism before the upgrade activates, or risk losing access indefinitely. Sprout addresses begin with "zc," which can help users recognize old wallet records.

Regular ZEC holders using modern supported wallets do not need to migrate their coins out of Sprout, although keeping wallet software updated remains important. The blockchain also cannot reveal who owns the remaining Sprout balance, meaning nobody knows how much belongs to active users and how much is already unreachable because its owners lost their keys. Once NU7 activates, even the actively managed portion will join that unreachable category.

Meanwhile, NU7 brings a benefit for everyone else: Zcash's target block interval falls from 75 seconds to 25 seconds, meaning transactions should receive their first confirmation approximately three times faster, according to the Zcash Foundation.

For Sprout holders, however, the number that matters is block 3,543,405, expected later today. Once the network reaches that height, any remaining coins in the old pool will be frozen under the new rules.