2 year old gist proof silently stopped working

2 years ago (June 17 2014) I proved my GitHub identity for https://keybase.io/timmc identity with https://gist.github.com/timmc/d78ac17dcec759742d04. Today I was [startled](https://github.com/keybase/keybase-issues/issues/1990#issuecomment-179626958) to discover that the proof was no longer considered valid.

I was told in the original email that

> We'll periodically check your proof every few hours. If it ever fails repeatedly, we'll let you know.

but I never received such a notification. Are notifications in fact enabled? Was there a period where they were not? Have requirements for proof gists changed out from under me?

(I've since added a new proof gist.)

Start with the linked proof gist and the referenced discussion in issue #1990, then establish whether the proof was invalidated, requirements changed, or a notification was missed. Done means documenting the cause and confirming the notification behavior.

Written by the indexing model from the issue text.