SIMD-0313: Drop unchained merkle shreds

SIMD-0313 (Drop unchained merkle shreds) is a Core Standard Solana Improvement Document by Ashwin Sekar; proposal status Activated.

Active on mainnet-beta since epoch 821; active on testnet since epoch 811; active on devnet since epoch 914.

Feature gates

• Feature gate: drops unchained Merkle shreds #2149
• Related gate: Enable chained Merkle shreds #34916

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