set status_server [<text>]
This variable contains the string displayed in the status bar when the client is connected to a server in the current window. If the client is not connect to a server in the current window, nothing is shown.
This string is represented by %S in the STATUS_SERVER string. If the expando is used inside STATUS_SERVER, it expands to the server name.
Customizing the current server indicator:
/set status_server Server: %S