Many people have brought up this topic before. There are many reasons why disabilities have not been brought to the Sims 4. When it comes to physical disabilities, what is perhaps one of the main reasons they are avoided is due to how the worlds have been created. They are only semi-open for exploration and are all made for those who are able to walk without problems. This means vehicles in general are near impossible.
While they did introduce bikes, the Sims can only ride them so far and, as University has shown, stairs often get in the way so your Sim has to dismount, take the stairs and then mount their bike again. That won't work with a Sim who needs a wheelchair or even a walker. Next is the need to do new animations. The team would need to create new animations for wheelchairs, walkers, crutches, canes and the like. There would also need to be new animations for things like leaving a wheel chair to enter stationary sitting surface or a bed as well as doing careers where the Sim has to stand up or ask for help from another to reach or use an object. There is also the matter of certain clothes clipping through the aids.
Prosthetics could be possible. Arms and hands would likely have to look realistic in terms of shape and work as a skin swatch, but for prosthetics like hooks for hands would be unlikely. Legs would probably be able to work like how boots do, so those could likely be more diverse in construction and not limited to looking exactly like a "perfectly-shaped" leg and foot. There does rest the likely problem of someone wanting their Sim to be able to remove their prosthetic, however...
There are then the "experience" that could actually cause a problem. With how Sims work, gaining empathy for someone physically disabled doesn't quite strike me as happening, since Sims tend to route to places on their own. A person in real life knows the difficulty of looking for routes optional for them. In the Sims 4, they either likely won't have that problem because they will be programmed to go only on certain routes for their disability. More likely, they will try to take the most ridiculous route and find they can't go there, then you'll try to make them go on the route they can use, but then they suddenly turn around and seek a different one that is all the way across the lot...
Meanwhile, mental disabilities, disorders and traits are a can full of worms. As it goes, those are sensitive topics and fall into spectrums. There is no "one fits all" in the matter and can lead to a lot of upset Simmers. The "erratic" trait alone is a good example of how people would react to such things being introduced. "Not all people who are [insert label] act like this!" "This was done insensitively!" "The word you used to describe this trait is degrading!" "I'm [insert label] and this doesn't represent where I fall into the spectrum." (I've had these problems with some regular traits in the Sims 4 not being what I expected them to be, but those are somewhat easy to brush off. More serious things, however?)
Lastly, despite the Sims 4 is a life simulator, it isn't a very serious one. They try to avoid the more serious and grave things in life and when they do address them, they often do it via humor. Almost all of the deaths have a somewhat goofiness to them. Animals cannot die from anything other than old age. If a Sim is injured, it only shows up as an uncomfortable moodlet. Certain words are censored with things like "whoo-hoo", "juice" and "nectar"... So if the Sims 4 were to ever add in any of these inclusive things, they would probably try to make them a bit light-hearted. Unfortunately, trying to make them light-hearted is exactly what upsets people.