This is an interesting YouTube video (only a minute long) that talks about ways to have disability in a D&D setting, but could be adapted to other fantasy settings.
A comment I left there:
Another way to do it is with the mechanic of "curses that cause damage that can't be fixed." In my main fantasy series, in addition to magic being unable to fix congenital issues, some curses and even some kinds of magic cause damage that cannot be fixed even after the curse is dispelled.
There is also a condition called "magic burn" that is basically "too much magic use over too short a period of time can cause something like radiation burns." Especially with combat magic. This condition, if not treated in time, can do anything from causing lesions on the skin, to nerve damage, muscle damage, or even death. How easy it is to get magic burn depends on your skill level and giving your body time to work up to a certain level. That is, even if you learned how to do an advanced spell very early on, using it or just using it too much can be deadly at a low level even if it's not deadly at a higher level, because the higher level mages have had longer to adapt their bodies to the higher magic amounts. And if you get magic burn, the treatment involves removing the magic from your body to stop making the damage worse. Yes, some of the damage can be fixed with healing magic later once your body has had a bit of time to recover, but sometimes the damage is too extensive to fix. Especially if performing too difficult of a healing spell was the cause of the magic burn, then more healing magic would just make things worse.
Also there's "magical exhaustion," when your use of magic has exhausted you physically and/or mentally enough to hurt you. It's a lot harder to die of this condition, but not impossible.
Another thing I use is that magic can't fix delicate structures like eyes and the inner ear, or else any fixes done to those parts is limited, or the magical surgery to fix it is super expensive because the number of people with the skills to effectively do such delicate work are few and far between. This is how I justify having deaf people in my fantasy setting. Well, that and some people are born deaf, and again, magic in that world can't fix congenital issues. (Well technically it can, but it takes years of regular sessions and is super expensive unless you've got a friend willing and able to do it for free. The skills to do so are not common, though.)
~ End comment ~
Obviously I was talking about my Ravenstone series. This gets mentioned as early as like, the first or second chapter of book one, after someone sees baby Dalia with her congenitally missing left foot. Her parents mention it
could be "fixed," and they know someone who would do it for them for free, but the process takes so long that she would already be used to functioning without it before any changes would be able to manifest.
I will also add that while it
is sometimes doable and
relatively easy to regrow a lost limb, the process of losing a limb you previously had (if no curses were involved) takes between a few days and a few
weeks. This is because, while it is technically possible to do it in less than an hour, the process is tortuously painful if done too quickly, and some people have even died from the stress of it. So it gets done slowly when possible, to make the pain more bearable. Also, on Earth the level of magic available for witch use varies by location and by who is in the area... some people's bodies generate a lot of magic that can be used by other people as well; on the other end of the spectrum, some people don't generate much of their own magic and are largely dependent on ambient magic for spells past a certain threshold. So whether you even have the magic available to attempt to regrow a limb depends on those factors. Desert areas like the American Southwest are notorious for being magic dead zones largely dependent on human-body magic generation, reservoirs of magic deep in the earth coming from other parts of the world (which take special practices to access), or open portals to one of the faery realms if you want to do anything as magic-heavy as any kind of healing magic. (Because life, especially sentient life and ESPECIALLY sapient life, is what generates magic, at least in that part of the multiverse. Humans and faeries and other sapient beings capable of tool use and civilization generate the most magic of all.)
Oh and I haven't even gotten, in the books, to trying to repair or replace entire organs with magic. Though thinking about it now, I think the easiest thing would be to do a mundane organ transplant and then use spells and potions to keep the body from rejecting the organ. Or, if you have enough time and money, have someone use blood alchemy to grow a replacement organ from your own stem cells.
And like the original video says, some conditions are either with you at birth or come later because they're in your DNA. There's no magical cure for Ehlers–Danlos syndrome, for instance; one character who has it, the only thing they can do is use spells that either help keep their skeleton in the right configuration, or if something happens like their shoulder popping out of its socket, using a spell to put the bone back into its socket. Another character has epilepsy, and there doesn't appear so far to be anything magic can do for her, apart from cushioning spells to keep her from hurting herself during a seizure. Asthma is another one that doesn't have any magical treatments.
And, naturally, there are no magical cures or treatments for most mental illnesses or differing neurotypes. Some potions exist for anxiety or depression, but most of those are just using magic to make mundane ingredients / medications more effective, and/or to stretch out the supply of a mundane medication (put one pill into a gallon of the right potion, and you effectively turn one pill into 100 doses of potion with the same strength as the original pill, so I can see witches in the US buying exactly one pill of some expensive drug and using said potion to have enough of the medication to last a few weeks or months.) Mundanes are much further along with treatments for mental illnesses than witches are, so witches usually just use the mundane solutions there. Hell, a lot of illnesses have better mundane treatments than magical ones. Magic, in the medical field in general, is best used for healing injuries and fixing things done by other spells.
Oh and I almost forgot: healing magic works best if the person being healed A. Isn't fighting the process, and B. Is focusing their Will on helping the healing process. So healing magic is more effective on older witches who are cooperative than it would be on, say, an infant or other young child, especially given that healing magic often causes pain while it's happening, if for no other reason than the tissues being moved around to heal them effectively. So even if you opted to try to quickly "fix" something like Dalia's missing foot when she was an infant or toddler, her young Will would work against you because all she'd know is she was in pain. Such resistance isn't as effective in kids who haven't started doing magic unintentionally yet, but it's still a stumbling block.