From Gregory Maxwell (Bitcoin developer) on the OP comments:
> That signature type didn't exist until after Hal's was out of comission, so it was presumably created by someone who obtained hal's private keys after his death FWIW. You can see that address was actively sending transactions long after hal's death so unambiguously someone else has control of the key.
> The signature you posted isn't compatible with the Bitcoin blockchain, it's a new signature type we introduced specifically for message signing which was first released in Bitcoin 0.5.0 on November 1st 2011. But the specific format that it's in is an electrum style which wasn't even proposed until mid 2013 -- https://bitcointalk.org/index.php?topic=218471.0 I'm not sure when it was first implemented. By the time signmessage was created in late 2011 Hal was profoundly disabled and only able to use a computer with someone elses help, it wasn't widely used until years after. Other commenters have pointed out that this address was active in 2017 so that gives an obvious explanation: The message wasn't signed by Hal but whomever is using his keys now.
From Gregory Maxwell (Bitcoin developer) on the OP comments:
> That signature type didn't exist until after Hal's was out of comission, so it was presumably created by someone who obtained hal's private keys after his death FWIW. You can see that address was actively sending transactions long after hal's death so unambiguously someone else has control of the key.
> The signature you posted isn't compatible with the Bitcoin blockchain, it's a new signature type we introduced specifically for message signing which was first released in Bitcoin 0.5.0 on November 1st 2011. But the specific format that it's in is an electrum style which wasn't even proposed until mid 2013 -- https://bitcointalk.org/index.php?topic=218471.0 I'm not sure when it was first implemented. By the time signmessage was created in late 2011 Hal was profoundly disabled and only able to use a computer with someone elses help, it wasn't widely used until years after. Other commenters have pointed out that this address was active in 2017 so that gives an obvious explanation: The message wasn't signed by Hal but whomever is using his keys now.