set status_insert [<text>]
This variable sets the string displayed when the client is in insert mode on the input line. When the client is not in insert mode (i.e. is in overwrite mode), nothing is displayed.
This string is represented as %I in the STATUS_FORMAT string. The %I is treated literally in STATUS_INSERT.
Customizing the text displayed when in insert mode:
/set status_insert (Inserting)