If you tell them it's a story as soon as they are old enough to understand the difference between facts and stories/myths/legends, would that not avoid the emotional reaction which comes from their eventual disillusionment? This approach avoids the minefield of "shh! it's a secret" . Is the belief that it's really a magical being rather than your family who give you gifts at Christmas worth all the suspicion, putting older siblings and classmates in difficult positions, and parental worries about how DCs will react the year they work it all out?
A child who gets the love, attention and goodwill of their parents and family (plus prezzies, kids' TV specials and time off school) at Christmas is going to find it just as much of a wonderful time of year without needing to believe in an actual living being called "Santa Claus".
On a side note, is there any sort of expert guidance for what to do about Santa if your dc is on the autistic spectrum? With a tendency for taking everything literally, an eye for inconsistencies to regular social patterns ("lies are wrong", "you can trust your mum/dad/teacher") it would appear to me that feeding them any sort of fairytale as truth could do major harm unlikely with a NT child once their upset has faded.
I read up when a friend's sister was diagnosed and one piece of advice was to never say "there are no sweets left" to a child with ASD, when there's a chance they may find the stash; instead, you must say "you can't have another one" as they will not grasp the pragmatic meaning of the first statement and read it as an outright lie, making them wonder why adults are allowed to tell them lies after saying "don't fib". If something this small can be problematic does "Santa exists" cause even more problems?