And they still put in that cliffhanger?
It wasn't much of a cliffhanger, really. The finale resolved all the season's outstanding story arcs and left the characters in a pretty good place. The tag at the end was just a potential hook for a new villain, just in case the unexpected happened and they got picked up after all. So
Batwoman had much better closure than
Legends of Tomorrow, which gambled on a huge cliffhanger and lost.
Shows have been unexpectedly uncancelled before, like the sitcom
Sledge Hammer, whose first season ended with the characters and their city all being blown up in a nuclear explosion because the producers assumed they were cancelled, and then they got renewed and had to scramble to undo that decisive ending. They opened season 2 with a disclaimer that it took place 5 years before that season finale, thereby giving themselves room for 5 more seasons -- only to get cancelled halfway through season 2.
Nothing is absolutely certain in this game, so it's usually a good idea to allow for more than one possibility when making future plans. Rather than assuming for certain that your show will or won't be renewed, it's better to make plans that allow for both possibilities, just in case. Joss Whedon always tried to end seasons of
Buffy the Vampire Slayer and
Angel with episodes that would work as series finales if the show was cancelled, without closing the door for a continuation if it was renewed. I've always thought that was the wisest approach.