How to prevent users log in into the Devuan system and how to postpone system shutdown?

Sometimes you need to lock system and don't let users to log in. One of the solutions is to schedule system shutdown and postpone it with a lock from new logins. It can happen in case you want to upgrade the system, but it's impossible to kick logged users from the system and you want just to let them finish their work and then do you work.

Just imagine you need to postpone shutdown for 20 minutes and let users know that system maintenance is coming. Let's start postponed shutdown with an announcement, what is going to happen:

$ shutdown +20 "System upgrade is coming. Thank you for your patience"