"or presenting an obituary then waiting for 21 days" this part feels a little loose to me. Say you have a falling out with someone and forget to remove them as a successor, this could be socially engineered.
I'm a big fan of this feature, but I do think GitHub would do well to institute a "real name"/identity policy for successors. That won't be infallible either, but it's another hurdle for a would-be fraudster to climb over.
> Once we have received your request, we may follow up with a request for additional information, such as a copy of your photo identification, copy of the death certificate, and documentation confirming you are authorized to act in relation to the deceased user’s account, to verify that we are properly authorized to process your request.
I would imagine, but I do not know, that the successee would recieve an email allowing them to cancel the transfer within this window.