STRIX game server has no feature for the maintenance, unfortunately. Please consider running a separate server to manage the maintenance states to implement the requested feature.
If you stop all your (master and room) servers, clients become unable to login and all connections fail. You could show a “maintenance notice” triggered by a connection error to the master server, but it will be difficult to distinguish the case from other errors (such as a temporary network failure that retries will likely connect).
If you stop all your servers, all players are disconnected, but it is again difficult to distinguish from other disconnections, and such a “sudden disconnection” can be undesirable in some games.
So, we need to say it is not easy to enter to a maintenance state only using STRIX server functions.