https://webtan.impress.co.jp/e/2009/06/16/5880
ErrorDocument 503 /maintenance.html
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_URI} !=/maintenance.html
RewriteCond %{REMOTE_ADDR} !=192.168.0.4
RewriteCond %{REMOTE_ADDR} !=192.168.0.5
RewriteRule ^.*$ - [R=503,L]
</IfModule>
https://kantaro-cgi.com/blog/web-server/htaccess_maintenance_mode.html
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !\.(css|jpe?g|gif|png|js)$
RewriteCond %{REQUEST_URI} !^ment.html$
RewriteRule ^.*$ ment.html [L]
</IfModule>