How can I compensate for past years of Zakat that I paid based on a fatwa permitting the deduction of usurious loan installments, whereas the current fatwa does not permit that?

Deducting debt from zakat is a matter of scholarly disagreement. The website adopts the Shafi'i madhhab, which does not permit such a deduction. The majority of jurists hold that debt can be deducted from zakat, with most of them stipulating that the zakat payer should not possess other surplus wealth beyond their basic needs with which they could repay their debt, while some do not impose this condition. If you acted upon a fatwa that permits deducting debt or an annual installment from zakat, there is no blame upon you for what has passed, and you are not obligated to pay for past years. As for the future, the preponderant opinion is that debt is not deducted from zakat, which is what you did with this year's zakat, and it is more cautious and absolves your responsibility.