I know a Pro Eventer who has a string of personal and client horses from BN thru 3*. They ride/show all of them. They CAN ride in any division (except explicitly ammy) and will usually choose the Horse or Open division. Some others do not. There are a couple rated shows near me that aren’t large enough to have those splits. They only have an Open Novice division (for example).
I give lower level dressage lessons. I am a “pro” and registered as such on USEA. However, I only started jumping (eventing) 5 years ago, and have young horses. I usually enter Open or Horse divisions when offered, although some ammies I’ve gotten to know ride made eventers and they and their horses are in full-training year round, spend the winters in FL, etc. They have a LOT more experience, training, and horses than I do! So, it is what it is and I just try to do the best for me and my horse to enjoy this great sport!