Unfortunately, people want kittens, just as it's harder to find adoptive parents for older children, it's harder for rescues to re-home their older cats. I say this as someone whose last cat was an older rescue.
Yes, people should go to rescue centres, but if you really want a kitten, you'll struggle because they are usually rehomed quickly. Rescue centres I have been involved with are often confronted with potential owners who only want kittens. These are the people who will be on gumtree when they can't find the perfect kitten at a rescue centre.
As someone who has never had a cat from a breeder, and wouldn't, I 'm still not naive enough to imagine that OP's friends who will be rehoming the kittens would be at the rescue centres if they couldn't have her kittens, and I am genuinely surprised at the number of PPs with experience working with rescue centres who think four cats would have been rescued if it wasn't for OP's irresponsible actions.
You could argue than any post of kittens is promoting breeding, because it promotes the desirability of kittens over older cats. You'd not have batted an eyelid if the OP had posted pictures of the one kitten she plans to keep when it was 12 weeks old, without the backstory.
At least we can see the mother, we've been shown the environment in which she gave birth. Most of the cute kittens we are bombarded with on Facebook reels and posts we have no idea where they have come from.
Breeding is still legal in this country. Perhaps it shouldn't be, but it is. And whilst it is, perhaps driving posters like OP away from openly posting about it isn't actually the best thing. If it's going on, I'd rather see it instead of only seeing the cutesy results that don't tell any of the story whatsoever.
Posts like this don't encourage potential breeders any more than the myriad of cute kittens reels, pictures, memes and videos do. The myriad of kitten reels, videos, etc, foster the desire for kittens, yet I imagine a post of a kitten wouldn't have garnered anything like the negative response.
When people post their pictures of their adult cats on The Litter Tray, are you all demanding to know where the cat came from? If someone posted pictures of their beloved Burmese, would there be this big pile on about pure bred cats? Would you all be hounding the poster to know where the cat originally came from? If someone tells you it's a rescue, will you be treating them to the same level of disbelief OP has been subjected to? Yet pedigree kittens seldom turn up at rescues.