This is one of the undocumented “features” delivered in SU5. What it is really saying is that if you are not using Multiplayer, then you are disconnected from all of the Multiplayer servers. You are still connected to the MSFS servers for non-multiplayers. And the ability to change servers is only if you are using Multiplayer.
When I first saw this after SU5, I didn’t know anything about it. I just assumed the servers were overloaded and restricting users. I exited figuring I couldn’t use MSFS. The next day the same message appeared. So I exited again. The same message showed up on the third day. If the servers were unavailable for three days, the forum would be filled with messages. I ignored the message and went ahead with my flying. Asobo needs to document these new “features” so users aren’t wasting time waiting for servers.