I have a 1997 ProLine WAC with a 1997 Bravo 2 LX. I have a new still in the box complete raw water pump as a backup if needed.The question is should I take the impeller out of it, since I may not use it in the near future?
rubber parts go off of calendar years not how much you use it. you will most likely need a new impeller by the time you need to use the spare raw water pump.
Yes, remove it and put it in an air tight bag, it will last many years longer.
Note that because it has been installed, place a mark on it so when you do want to reinstall it. The vanes will be bent in the same direction that they are now.
Depending on the impeller quality and the material it was made from, probably all vanes will flip open to their original position when out of the pump...