To be temporarily unavailable is it harmful for a website?
This may be the case if the situation is not properly managed, which means that we know in advance that the site will be unavailable. If this is not the case, webmasters may think that the site, if not very important, is closed and remove backlinks. Similarly, robots of search engines can return a negative signal. If the outage is expected, the ideal is to return an HTTP code 503, which is defined for this situation.