I would agree that for many cats — well, particularly adult cats — it’s similar reasons for abandonment no matter what their ancestry.
I do think for some breeds, people did not do their research in advance , or did not take seriously what they were told about the needs of the breed. A big issue for Persians , the breed most often found in shelters in the U.S., is that people did not keep up with the grooming. Or people get a Siamese or Oriental and then complain about the cat being too vocal, too active or too needy. Similar with the Bengal.

The percentages of purebred/ pedigreed cats in shelters may be difficult to arrive at, especially considering how many cats in shelter get labeled as breeds they very likely ( or definitely) are not.
In some shelters every blue cat is a “Russian Blue”, all largish size longhair cats ( or sometimes just the large longhair tabbies) are “Maine Coons”, white longhairs are “Turkish Angoras” and nearly all the pointed shorthairs are “Siamese” despite the fact that the gene causing this pattern has been spread through the random-bred cat population over the course of 100+ years, and there are many pointed moggies whose most recent Siamese ancestor was likely decades ago — which would make them way less than 1% Siamese. Similarly, long-haired cats with a pointed pattern are very often labeled as “Ragdolls”, “Balinese”, “Himalayans” or “Birmans”. ( Actually sometimes they’re “Siamese” too.)
Spotted tabbies are often called Bengals and Egyptian Maus although the spotted tabby pattern has been in domestic cats all over the world for a long time.
Interestingly many of these breeds — including Siamese ! – are less popular than the Sphynx but you don’t see shelters filled with Sphynx.
I understand why shelters do this, either out of misunderstanding, perhaps thinking cats are like dogs and either a breed or a mix of breeds, or just putting a breed label on a cat to attract adopters. But it does cause confusion as to the number of “purebred” cats in shelters.

Here is a link with stats from the American Humane Association. This mentions Domestic Shorthairs are 95% of pet cats in the U.S. (I’m guessing surely that must mean the Domestic Longhairs too; I know they’re a smaller percent than shorthair cats but surely more than 2% or so. IF that does include both shorthair and longhair moggies, that would make the “purebreds” 5% of pet cats.

I’ve seen another statistic from a survey that said that the purebred cats were 3% of “owned cats”.

In dogs, purebreds are estimated to be 53% of dogs in the U.S., and about 25% of dogs in shelters. If there’s a similar ratio in cats, that would make it about 1.5% to 2.5% of shelter cats being purebred/ pedigreed cats.
I would not think there is a strong reason to believe that in cats the percent of purebreds entering shelters relative to their portion of the population would be any higher than it is for dogs; however, there is good reason to think it could be lower since many of the cats who end up in shelters are ferals/ “community cats” ( who really should be TNR’d and not rounded up and killed) So I would guess a figure of 1% may be closer to reality, though of course it would vary by location.

We did adopt an 11 year old seal point cat who, judging from not only a pointed coat pattern but his body type, coat type, deep, very Siamese voice, may really have been a purebred Siamese of the old type or at least . The people who abandoned him had said they got him as a kitten with “papers” from CFA. Their reason for dumping this poor cat was that it was their son’s cat but the son had now gone to university and these parents just didn’t feel they wanted to bother with a cat anymore. They’re the ones who had made the decision to get this cat at a time when their son must have been a little boy in primary school. This cat was so sweet and social, and handsome, but people did not want such an “old” cat and he was about to be killed the next day. (My friend worked at the shelter which at that time, 20 years ago, had a much higher death rate and could not stand to kill this cat so he brought him home.

( BTW,this cat was declawed, but I don’t think that had anything to do with why he was dumped since he did not have any behavior issues that sometimes are associated with that abuse. It was just another example of these people’s callous attitude toward a wonderful cat. ) Luckily we had him for 7 more years. 🙂

I have seen quite a few examples of pedigree cat owners and breeders who really do not give a hoot about their cats per se. This explains the prevalence of inbreeding in the quest to maintain a “look” or aspire to the standard. This is the thinking behind the pedigrees which certainly ensure inbreeding and proliferation of genetic problems. That doesn’t matter. What is important to them is the glamour and self-satisfaction of their cats winning all kinds of rosettes, cups, championship status , etc. They think that this reflects on their special abilities as a breeder or owner when in fact they are just using their cats as pawns for self-glorification. This has gone wildly wrong in several breeds including the Turkish Angora and the Persian. All their hard work has produced cats that are far less attractive than the originals. If their cats are not up to scratch vis a vie the standard and don’t do well at shows, they are eventually disposed of or just disappear off the map. I assume this occurs quite often because there are societies that specialise in pedigree cat rescue.