Welcome to NR!
An 'outline' doesn't really exist, or at least, is a misleading phrase I think. It's got little to do with where the horse's head or even the horse's hind end but about how well the horse is able to use his back. It's not something that is confined to a school either. For all of you saying you hack, chances are, you're halfway there if the horse feels forward and can walk, trot & canter in reasonable balance, doesn't hollow his or her back and is able to find his or her balance up & downhill
When you think of the components making up the scales of training are rhythm, suppleness & contact as the first three, nothing is about the 'outline' of a horse. But when they come together, the horse will be working through his back and will naturally be working in a biomechanically healthy way.
Part of the trouble from a RS perspective, I think is that to have a horse that is going to remain sensitive eenough for a rider influencing the rhythm in their own body for example, is probably going to be too sensitive to teach a complete beginner who will wobble all over the shop initially. And it's mainly a question of economics for them, you might only have a couple on the yard and a yardful of steady horse's that know the drill & will keep riders safe but not necessarily teach them the finer details of riding.
It will also depend on the RI's experience too. Sadly not all RIs have been trained using the scales of training themselves, as in this country particularly, a lot of emphasises is placed on competition success rather than necessarily fundamentally correct training. The two can be exclusive but are not always so
I've ridden a high level dressage horse that had been successful that could barely use his back to walk up a hill, as he'd been essentially 'held' in a false way of going by strong arms and strong legs.
Those horses & RC's do exist that teach more classically but it's generally much more expensive experience
I was really lucky as a teenager to work for a classical minded instructor & it's certainly helped - so I would recommend some schoolmaster experience if you can, it's well worth it!
You absolutely don't need to ride in an 'outline' to own a horse- in fact, not doing so is probably healthier than forcing an outline in a horse.