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In reality, whilst the Merkle Patricia Trie is implicitly pruned, the BONSAI feature did introduce an extra data structure: the Trie Log (more detail about BONSAI and trie logs here: https://consensys.io/blog/bonsai-tries-guide)
The Trie Logs are retained in order to cope with chain reorgs. After each block is finalized, Trie Logs older than that are no longer required so it is safe to remove them from both the node's and the network's point of view.
How?
If you want to use this feature before it is enabled by default, simply add this option to your besu command: --Xbonsai-limit-trie-logs-enabled
When you restart besu it will begin pruning, block by block (and a cheeky bit during besu startup).
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