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Yes, there are multiple policy rules that are not configurable. You would have to change the code and recompile to accept infringing transactions to your mempool.
A simple example would be a transaction with two OP_RETURN outputs
Could you mimic the effects of a soft fork solely by tightening mempool policies?
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Not really. Making mempool policy stricter may eventually cause blocks to exclude transactions infringing on the new rules as hashrate updates. However, as long as just a small portion of the nodes (perhaps as little as 2–5%) run with the old rules and at least one miner has not upgraded, the transactions would still appear in blocks.
On the other hand, a softfork enforced by a majority of the hashrate would actually reorganize offending blocks out of the best chain since it would actually be a consensus rule rather than a policy rule.
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