Just wondering after baby outgrown the bath tub, do you still keep it or throw/recycle/donate?

You can keep it for other uses. It can be a toy for your child to play with (I remember using mine as a make believe boat when I was a kid. If not, you can use it as a laundry bucket for handwashing or soaking clothes. I wouldn't throw it and donating is fine if you know someone who doesn't mind using it. But best is to keep it and use it for other purposes.