Difficult one. I don't believe that any breeder who is in any way a good caring person will breed litters for Christmas sale. I can see why many rescues won't rehome during the Christmas period, especially those who are wholly or mostly run by volunteers as they are entitled to time off too and shouldn't have to be available for home checks, advice etcetera. And of course every puppy should be a much wanted and seriously considered addition to the family and not a treat or a surprise, BUT there are many people like myself who only ever have quiet Christmasses, have had dogs before, and would find it very easy to have a very successful puppy (or rescue dog) friendly Christmas. Down here in the south, its the norm to have mild winters and the recent summers have been impossibly wet so no easier or more difficult to housetrain. Also if I was looking for a dog of any age (which I am not) then I would take the right dog no matter what the time of year. We got my current dogs in mid december which meant that my husband was at home with me for the first three weeks we had them (I had retired and he had not) which was a huge bonus and may also be a consideration.
As I have said, I do get why rescues cease rehoming for the Christmas period, but given that they are always full to bursting at that time, maybe there is an argument for rehoming appropriate dogs to appropriate pre approved homes?