set status_scrollback [<text>]
This variable sets the string displayed in the status bar when the window is using the scrollback view. Because the memory usage of a window that is in scrollback view grows without limit, it's useful to have this visual clue to remind you to exit scrollback.
This string is represented as %K in the STATUS_FORMAT string.
Customizing the scrollback indicator:
/set status_chanop [Scrolling]