It should be available to anyone as long as informed consent can be achieved and they're of sound mind in the view of at least a few medical professionals.

I think it should be available as a medical directive, like a DNR order with specific criteria, and require several doctors to evaluate the criteria unanimously, and no family to object if the patient can't give informed consent, only whatever form of consent they can give.
It should be called off if the patient objects, regardless of their ability to give informed consent.
Scenario I'm picturing is a person with dementia who previously filled out a form stating that if they're no longer themselves or able to function, and other criteria they specified beforehand, and doctors agree the circumstances have been met, and the family doesn't object, it should be able to proceed even though someone with advanced dementia cannot consent because they cannot fully understand. If they say no it must stop.

I feel concern about people with mood disorders seeking that route, which is why I want a medical professional to say they're of sound mind.
Ultimately it's your life and your body, so you should be able to have that autonomy, but I think it's responsible to pause if a doctor says you're not in a rational place to make that type of choice.