On 12/08/2015 09:12 AM, Gavin Andresen via bitcoin-dev wrote:
> Stuffing the segwitness merkle tree in the coinbase

If such a change is going to be deployed via a soft fork instead of a
hard fork, then the coinbase is the worst place to put the segwitness
merkle root.

Instead, put it in the first output of the generation transaction as an
OP_RETURN script.

This is a better pattern because coinbase space is limited while output
space is not. The next time there's a good reason to tie another merkle
tree to a block, that proposal can be designated for the second output
of the generation transaction.