Add an option in "Settings > General" with a meta box to put the site in maintenance mode and add a message that uses the site's theme.
I've wanted this before, but "add a message that uses the site's theme" is a difficult requirement to fulfill 100%!
Maybe is easier to put the site name and a personal message.
Maybe we can check if an user is logged in and then Redirect 303/307 (?) to a configurable url (old site, html page)...
There is actually a built-in maintenance message in WordPress Core whenever the temporary file /.maintenance exists (e.g. during Core updates). This is extremely bare bones because its goal is function over form -- there literally needs to be a way to prevent files loading when Core is being updated.
Anything beyond such a minimal function is risky, and risks overstepping the bounds of CMS infrastructure. Even thinking "something is better than nothing" can end up being untrue, because the more features you add to Core the more potential issues with design, assets, security, performance, etc.
I'd possibly support a very minimal Maintenance Mode, but probably only if it avoided database queries and UI markup.
/* Maintenance Mode Functions */
define('MAINTENANCE_MODE', false); // default = false
But at the end of the day, its still code bloat. Many or most users probably will not want it or need it. What's the point of removing e.g. XML-RPC and other code bloat if other code bloat is put in its place? Etc.
I think using the given core implementation with slight override changes would be a good thing. Like adding a button to the menu bar to turn on/off. And letting people know the site is in maintainence mode. Reason being not every update/upgrade works as planned, with on/off switch, you would let visitors know AND you can finish up with being too hasty.
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