Of course it's too late to amend tax returns for years before 2015 to claim a deduction if your contributions would have been deductible. You could potentially file late Forms 8606 for each year that you made a contribution to report the contribution as nondeductible. It's likely that you'll be billed a $50 late-filing penalty for each late Form 8606 filed. (The late filing penalty is likely intended to cover the effort the IRS must expend to confirm your nondeductible contributions.) Without the Forms 8606 having been filed (the first possible year would have been 1987), the IRS generally will not give you credit for the basis.
Of course it's too late to amend tax returns for years before 2015 to claim a deduction if your contributions would have been deductible. You could potentially file late Forms 8606 for each year that you made a contribution to report the contribution as nondeductible. It's likely that you'll be billed a $50 late-filing penalty for each late Form 8606 filed. (The late filing penalty is likely intended to cover the effort the IRS must expend to confirm your nondeductible contributions.) Without the Forms 8606 having been filed (the first possible year would have been 1987), the IRS generally will not give you credit for the basis.