Some games officially announce when they’re going into maintenance mode. (And some games then update more than some non-maintenance titles, but that’s an exception.) But an awful lot of games just… stop having more than new occasional events and are very clearly not being updated, yet they are still available for play. You can go play one of those soft maintenance mode games right now, if you want.
The question is… would you want to?
There are lots of reasons to not want to; for one thing, there’s always the risk that these games could be shut down suddenly and without much warning, and for another you’re looking at what is basically a finished product. Starting fresh can sometimes feel like a fool’s errand. At the same time, examining these games as a fresh player can be an interesting or engaging experience, and you might even find a game you really like even if you wish you’d gotten into it earlier. So would you start playing an MMO in soft maintenance mode? Or do you stick to games where the lights are still on, so to speak?