Each year that you do not claim a tax deduction for contributing to a traditional IRA you must file a Form 8606 to report the basis to the IRS. You can fill out a Form 8606 for each year and send to the IRS. Since (I am assuming) you did not deduct the IRA, your conversion this year can be tax free if you report the basis this year. An individual who fails to file Form 8606 to report a non-deductible contribution will owe the IRS a $50 penalty, but you can request an abatement of the penalty for reasonable cause.
See Instructions for Form 8606 (2021) and Find prior years forms, instructions & publications.
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