At my barn we do it because show season has ended and going back to basics is key over the winter, when you ride with stirrups we take advantage of it and use little shortcuts. Riding without stirrups really forces you to do a number of things, you have to be connected with you horse because you’re very vulnerable at first, it builds confidence, if you can ride without stirrups you can do anthing, your leg and core will get the workout of your life, you are reminded of the correct and best position to have the most balance possible.
I love riding no stirrups and do it all the time in my lessons. I feel like the horse is listening to me better and we just work super amazing with no stirrups. Then when I pick them back up we are golden
But you should ditch stirrups all the time and not just bc no stirrup november. I personally love going without, especially when Im not liking how the horse or me is doing as you can really focus on riding the horse and not worry about yourself that much like posture and that stuff