The first comment in the pull request is: "The maximum size for OP_RETURN outputs used to be 80 bytes, then got changed to 40 bytes to be on the safe side. We h

The first comment in the pull request is: "The maximum size for OP_RETURN outputs used to be 80 bytes, then got changed to 40 bytes to be on the safe side. We have now been running with 40 bytes for about 9 months, and nothing catastrophic happened to the Blockchain, so I am proposing to increase it back to 80 bytes." The commit that is referenced by this comment is "script: reduce OP_RETURN standard relay bytes to 40" https://github.com/bitcoin/bitcoin/commit/8175c790eb https://github.com/bitcoin/bitcoin/commit/8175c790eb There you can see that it was originally 80 bytes.