In a coinbase transaction the second transaction output seems always to be a 32-byte witness reserved value. Is there any constraint on this value?

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The only constraint on this value is that it must be 32 bytes. Otherwise there are no constraints. This is done to allow for a future soft fork to commit to new data and have non-upgraded nodes still be able to store something in the block that commits to the new data. As such, the witness reserved value must remain unconstrained to allow for a future soft fork to work.

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