What he said on Google+ here is sneak peak of it below :
Here’s one for fans of the hypertext HTTP protocol — should I use 429 or 503 when the server is overloaded?
It used the be that we’d only see 503 as a temporary issue, but nowadays we treat them both about the same. We see both as a temporary issue, and tend to slow down crawling if we see a bunch of them. If they persist for longer and don’t look like temporary problems anymore, we tend to start dropping those URLs from our index (until we can recrawl them normally again).
A 429 code means “Too Many Requests (RFC 6585),” “the user has sent too many requests in a given amount of time. Intended for use with rate limiting schemes.”