To figure out the previous year contributions for an IRA going back further than 10 years, you can work off of a formula. To calculate your IRA basis at any point in time, you would need to add up all nondeductible contributions you've made to date and subtract any nondeductible contributions you have withdrawn. Therefore, it would follow that you can subtract the total value of the IRA and the contributions that your broker recorded for the past 10 years, and the remainder is the prior years' contributions that you're looking for.
This shouldn't have anything to do with COVID. As long as it's reasonable, makes sense, and you can show where you logically got the number from, you're golden.
