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A 503 Service Unavailable server status code is to be used when the server is currently unable to handle the request due to a temporary overloading or maintenance of the server. It is a good method for temporary downtime and to help Google know that your site is not completely offline but rather, it is just a temporary thing.
Well, how long is temporary? John Mueller from Google answered the question on Twitter saying use it if your site is offline for hours. John said "roughly hours is fine" adding "if it's weeks then it's not really temporary." John later added that "it would be misleading to include it in search."
No, you do not want to mislead Google!
If it's not accessible for weeks, it would be misleading to include it in search, imo. It's an error page, essentially.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) June 8, 2017
John answered this before but he didn't go as far to say it is misleading.
Forum discussion at Twitter.
