Something that has become rather noticeable- the CW is no longer promoting Supergirl. It used to be at the end of the current episode, there would be a preview for next week's show. But outside of the trailer they released announcing it was coming back, there have been no commercials at all. Even the actress playing Kelly remarked about it on Twitter.
I get that the show is ending, and that even then, the ratings have been dropping for years. But why wouldn't you put the bare minimum effort in to promote the show? Maybe try to grab some ratings on the way out. Or at least try not to piss off actors you may want to try and get back for guest spots later on down the road.