Answered by Sidi Sulaiman Hart
I have heard that the consensus of the Shafiyya says that one who misses years of missed fasting and zakah is obligated to fulfill those as they are regarded as debts own to Allah. Is this this confirmed?
This is the consensus of all four schools.
How does one fulfill the debt of fasting if, say that person missed 10 years?
MAKING UP MISSED FAST-DAYS
il.33Someone obliged to make up some fast-days of Ramadan is recommended to do so consecutively and immediately.
It is not permissible for a person with some unperformed fast-days of Ramadan to delay making them up until the next Ramadan unless there is an excuse (N: for delaying). If one delays until the next Ramadan, one must pay 0.51 liters of food (def: h7.6(A:)) (N: to the poor) for each fast-day missed, in addition to making it up. If making up a fast-day is delayed until a second Ramadan comes, then one must pay double this amount for each day. And so forth: every year that passes upon an unfulfilled fast-day adds 0.51 liters to be paid for that day.
(Reliance of the Traveller, Nuh Ha Mim Keller)
Note: So if one had ten years of fasting to make up, you would have to fast for ten months. The food one would have to give in charity would depend on how many Ramadaans have lapsed since the missed fasts.
May Allah Forgive us.