A best practice while disabling entire site for maintenance is to render a HTTP 503 error along with a
Retry-After header which will help the to keep indexing your website after downtime is over.
Other things to consider:
✔️ Use static HTML with minimum off-page resources; using inline CSS is recommended
🔴 Do not block robots.txt with 503 error; Also do not allow robots to crawl entire site too.
🔴 Do not use any robots blocking techniques