What is the debt obligation of closed S Corp business

For the most part, that is true -- shareholders are only responsible for a corporation's liabilites to the extent of their own equity in the company.

However many banks/lenders, when dealing with small-business owners, will also make the individual sign a "personal guarantee" when they incur debt on behalf of a corporation. If you ever signed a personal guarantee for these debts, then those creditors may hold you personally responsible for repayment even if the company no longer exists.

There are other situations where you can still be held personally responsible for the corporations debts, mostly involving misappropriation, conversion or actual wrongdoing. For example if you knew the company was going under, and you ran out on a last minute shopping spree and maxed out the company credit card to buy things for yourself, you would probably end up having to pay for it out of your own pocket. (I know a guy who did this and then he went bankrupt; even in bankruptcy the credit card fought him in court and won).