23F is -5C which is considered quite warm by local winter standards. I will ride outdoors in -15C/5F as long as it’s not too windy. Wind chill makes a big difference.
I will have an indoor this winter for the first time in a few years and will probably be riding down to easily -20C/-4F. This won’t be a full on WTC work like in warm temperatures. The TC are shorter bursts, but the real trick is staying warm enough long enough to warm the horse up enough to trot. I find trot really warms me up.
All that said, we do, horse and human, have to adjust to the colder temperatures. In November -5C is brrr! cold for us humans. Horses don’t have to expend any additional energy to keep warm until it gets below -10C but they do need a couple of weeks to adjust. By January -5C is a nice warm day.