Let's break "disabilities" down:
-Minor physical disabilities. Glasses and hearing aids can just be added as CAS accssories. Disabilities that can be completely controlled by things like an inhaler would probably work best with a simple inhaler/pill bottle/etc object that, when in a sim's inventory, prompts an occasional relevant animation on their part. It'd be entirely cosmetic, but it'd also be overwhelmingly opt-in (in the few cases it isn't, I've never seen somebody complain about townies with glasses), and simple enough to add.
-Gross motor disabilities. Mostly these cover crutches and wheelchairs. In theory, you could handle these by just putting a relevant object in a sim's inventory, and creating a priority system. (So having both crutches and a wheelchair in inventory won't cause weird conflicts.) In practice it'd be a major beast to animate all that, and working around built-in game elements will also be a pain. (How will wheelchair users handle stairs? Will they require an empty seat at a table to roll up to - meaning a chairless spot that normal sims can't engage with - or will they do something to the chair to be able to engage?) I'm not intrinsically against, but practical implementation will wind up becoming a major headache.
-Invisible but practical physical disabilities. Anything that has a regular impact on your quality of life, but isn't either immediately obvious or can be medicated down to a negligible impact. Frankly, EA should just have a few more illness animations that mimic vague illness indicators (stomachache, headache, nausea, etc.) and let modders go to town making their own traits. There's no way EA can create enough or to meet everybody's idea of how these traits should be handled, so it's best to assist modders to create all the variety of traits that anybody could ever want.
-Mental disabilities. Hard nope. We live in a world where "autistic" and "retarded" are still used as insults. Some people will use those traits as punishment for sims they don't like, and/or heavy handedly applying them as the whole of a sim's personality. Other people will inevitably feel that the way the sims team applies these traits offends them. EA is smart to stay well away from this clusterplum.
I could see mental disabilities being handled as traits if the traits were vague and general enough not to point to any group of people in particular. (E.G: "Gloomy" for depression, "Erratic" as a catch-all because the name "Insane" already had to be changed, etc.) Anything more finely tuned than that, I don't trust that the sims are smart enough not to make a mess of it.
