In configuration.html, it is specified that srv_is_up should return a Boolean value - True if the server is in the ready state, False if the server is in the maint or drain state. In turn, this call returns 1 if the server is in the ready or drain state, and 0 if the server is in the maint state.
core.Warning("Server State: " .. txn.f:srv_is_up(backend_name .. "/" .. server_name)
Server backend/server is going DOWN for maintenance.
Server State: 0
Server backend/server remains in forced drain mode.
Server State: 1
Server backend/server is UP (leaving forced drain).
Server State: 1