Wordpress - Is there a maintenance mode in Wordpress core?

Yes there is. But it is primarily meant for short interruptions (such as during upgrades) and not very user-friendly. WP creates .maintenance file in root (with timestamp info about time) to trigger it and removes after it is done.

See WordPress Maintenance Mode Without a Plugin for reference.

In practice using a plugin is usually more convenient and functional.

There is no maintenance mode in core AFAIK. And as far as plugin's go, I prefer JF3 Maintenance Redirect as it allows IP address whitelisting.