Well the thread is pretty silly in that breeders don't 'cull' chocolate puppies.
They are usually placed in pet homes on neuter contracts unless the breeder keeps them in the line for other desireable reasons (eg good heart history in the line). Simply because a line may produce chocolate dogs now and then does not mean you eliminate the line -- as a matter of fact Susan Cochran who breeds in the US has chocolate genes in her lines (the dog pictured is from her website) and she simply places puppies when they show up in pet homes. They are rare and do not turn up very often, just like blue-eyed tris (we have one from Ireland on our board
). Also every now and then you get all black cavaliers and they too are not recognised though historically all black spaniels are well known amongst the 'small spaniels' that lie behind the cavalier line.
Like all breeds recognised by clubs you do have to decide on a breed standard and that includes colours. Remember that no breeds had officially 'fixed' looks and standards, including colours, until the formation of the UK Kennel CLub in the late 19th century. Breeding dogs before that was not a 'hobby' or a 'fancy', they were simply bred for whatever purpose they were to be used for, with breeds developed to suit a purpose be it hunting rats, dog fighting, retrieving, rescue, etc. There was no such thing as a cavalier, there was simply a small spaniel type that some people would have bred to a preferred type that they liked (hence the Duke of Wellington bred spaniels of the red and white variety, a colour that became known as blenheim because he lived in Blenheim Palace, named for a famous victorious battle).
I don't think chocolate is a colour that is desired in many, perhaps any of the spaniel breeds. It was not acceptable in the UK CKCS Club from the establishment of the breed standard and the US would simply have more or less followed that standard in the CKCSC and ACKCSC, so it isn't that in the 70s someone suddenly decided not to include chocolate cavaliers. The standards are all the same with tiny exceptions in phrasing.
Susan Cochran used to have a whole public section of her website about the chocolate line but she seems to have removed the links. I am guessing she was probably concerned about backyard breeders deciding to try and get choc puppies to breed with no proper health testing (and no reputable breeder would allow their good quality dogs to be bred with chocolates). Tis has been a problem with black cavaliers -- total BYBs charging extra for a dog not even recognised and from lines they certainly have not ever done even basic health testing on.
It is a shame about the Cochran chocolate pages though as they were quite interesting. I think she has even blocked them from being picked up by search engines though I believe the pages are still there.