Derpy

I only learned about this now, and only learned about the year of the event when I was done writing this comment, so I don't know how current Derpy's ableist exclusion/normalization is.

The thing that made Derpy "sacred" on that show, is that she put a dent in the oblivious perfection of the cast of a show that claims to promote acceptance and being inclusive. Yet, all the characters are young, beautiful, healthy, and mentally sound, and live lives humans could only achieve if rich. Their only flaws are being too much of a cute thing: shy, cheerful, vain, or short-tempered. You don't need inclusivity, acceptance, and tolerance, when everyone fits the same privileged mold. Those ponies are like a happy version of Doctor Who's Daleks. Hive-minded lumps of perfection who were each slightly modified from the same template. That is never a good thing, but rather it is why we need movements like ADL, BLM or Bopo: the preference of "desirable" template demographics directly hurts those who don't fit that template. The depiction of "rich, healthy, sound-minded, (white), skinny, young, and beautiful" as "normal" needs to die, not be reinforced.

Basically, the show is a cotton candy version of your average american "rich white suburbians whose worst problem is a DUI, and whose laughter tunes out the struggles of the less privileged" TV show. And frankly, I find those shows disgusting. They pretend that the world is perfect because only "perfect" characters with asinine vices get a voice and the rest is either trash or not written onto screen. I don't know what's wholesome about carefully NOT showing characters who represent the real people who could use more "wholesome". Fat people, LGBTQ people, disabled people, mentally ill, poor, people of colour,... All those who need inclusion don't get it on MLP: FIM BECAUSE THEY DON'T EXIST IN THAT WORLD, because apparently, their existence would harm its perfection. In other words, there is something wrong with them as their inclusion is so easily yet obviously avoided. Your cast is like that grinning white family in new, expensive clothes in front of a big, expensive house, on a stock image site.

Yes, you want to show kids a perfect world. But wouldn't a modern perfect world be one where those who need acceptance the most, are unapologetically visible and loved? There are two ways to achieve a perfect world: by offering equality to the underprivileged and the oprressed and having a society of true equality and universal quality of life, kindness, and compassion for those who most need it... or by eliminating them so only happy, privileged people are left. The creators of MLP: FIM made their choice in this matter when they "normalized" Derpy, a choice that makes my fat, female, disabled, mentally ill, ex-poor, half-Arab, biracial, Jewish ass, that can't relate to any one character beyond "female", cringe. Think about it. I am all these things, and these things, things SO MANY are, are only represented in MLP by biological sex...

Rainbow Dash has become a bit of an LGBTQ character, but only because fans forgot that the LGBTQ community didn't invent rainbows.
That is all. That is all the diversity MLP: FIM has mustered. That is all MLP has done to show the audience those types of people who NEED that acceptance and inclusivity the show claims to promote. Give a pony a rainbow design so FANS can project LGBTQ representation onto her.

Then came Derpy. She looked and sounded like she was on the slightly handicapped. People loved her not only for being funny, but for being portrayed as a lovable character who matters, "despite" not being perfect like the rest of the cast. The show, and anything else that influences children, needs MORE of that, but they made it less/zero. This sent a message, and this message is why the creators are getting "hate": because the message of making Derpy "normal" is a message of hate in itself. It conveys that in order to continue having a role on the show, you must be as perfect as the others, with perhaps the endearing flaw of clumsiness remaining. You must be smart and symmetrical, not cross-eyed or otherwise facially impaired, and show no signs of mental development issues through slow/slurred speech. 
It's not ableist to have a funny disabled character as long as the "funny" part isn't scornful and the character is treated with love and respect. Some handicaps make you prone to "funny" moments, just as others make you more likely to frustrate people or yourself. That's a fact, not an "-ism".

My hopes for a fat pony that shows kids that you don't have to be skinny to be popular, just died along with your only subtly handicapped one.

TL;DR: portraying disability with the same humorous approach as the rest of the franchise, isn't ableist. What is ableist, is to remove the disability and "normalize" the character in order for it to remain acceptable. Representation matters. Derpy was representation.

/inb4 "don't make MLP political": allowing everyone to feel welcome in the world they're stuck in, should be a non-issue, not "political" and not "agenda". It should just be the state of affairs.