How to implement safe maintenance mode on Minecraft server?

The purpose of maintenance is to limit new player entry, notify existing players, and verify normal access when the update is finished.

Step by step

Announce the maintenance time and the servers that will be affected. Prepare test accounts and backups required for return.
Open a whitelist on a simple Java server and add only the test team. In the proxy network, apply the login rule at the proxy layer as well.
Stop the server properly, apply the changes and enter the world with the test account. Complete data, permissions and plugin checks.
Turn normal login rule back on when finishing maintenance. Log in and query external status with a real player who is not on the list.

The status tool showing open does not confirm that the maintenance rule is closed, actual input is required.