Yes, it's legal. But you can apply for relief by filing a modification. You will need to present documentation as to your illness and homelessness and you'll have to make a good case for yourself, including your present intention to pay the child support you owe. There is no statute of limitations on child support so you will be on the hook until your dying day. Go to court and ask for relief. Depending on the judge you get, you may be granted some relief. It's worth a try. Do it ex parte.