Our tracks are shut in winter due to mud and risk of slipping. I am still unsure why a lot of people continue to track on unsurfaced tracks through winter with their horses in fetlock- knee deep mud... that can't be good for them either!
With the right management in place, most horses can have grass.
In Spring our track middles are left to grow to standing hay, and in winter our horses graze it, after the sheep have been in to eat all the green bits!
This is the first year the tracks have been shut so early due to the horrendous amount of rain over the last few weeks, and still to come, so we are being extra careful about the horses access to grass. They are only allowed out for a few hours am and pm, with the rest of the time spent on our hardstandings with ad lib hay in slow feeders.
In the winter the gates to the fields are left open, allowing complete free choice. At the moment, this will do to keep all happy and safe. IMO it's a lot better than stables, as they are still within the herd, able to move about and groom, socialise and forage as they please.