Your result
Spain — Diaspora & Religious Return
Possible
Worth establishing first, because a pending file and no file lead to different places.
Applications under the Sephardic law were handled through Spanish notaries, and each applicant received a reference number when the file was opened, followed by an acta drawn up at an appointment with the notary. If an application was made for you, one of those exists. Families frequently used a gestor or a lawyer who dealt with several relatives at once, so not knowing is common.
The way to find out is through the notary or firm that handled it, quoting your full name as it appears on your birth certificate. If a file was opened and is still pending, it is still being processed — grants have continued to be made years after the window closed.
If no application was ever made, that route is closed and cannot be reopened. What remains is the Civil Code concession, which is permanent: two years of legal residence in Spain instead of ten, and no requirement to renounce your existing nationality. It needs you to live in Spain, which the 2015 route did not, but nobody can take it away.
Either way, gathering your Sephardic documentation is useful — it is what a pending file may be waiting on, and what a new application under the Civil Code route would need.