14 hours ago, Ewgeniy said:Most likely you are right, but webmaster Yandex scares with terrible consequences if the code 503 is given. ))
It's only an SEO issue if it returns that permanently. Yandex doesn't know your planned maintenance, so warns you just in case it's an unintentional event/error 
From the RFC
Quote10.5.4 503 Service Unavailable The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.
Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection.