There is no maintenance, all the 'patches' are already loaded into your game install. They just flag the content on the fly, don't even have to take the servers down. And from what I understand even those flags are on autopilot. The only time I've seen the server go down was when they merged Teal server with Green server. This isn't a live server, all the content is already there, they are just using the time line to implement like the original servers did.
https://www.reddit.com/r/project1999...ssion_updates/