The statute of limitations for the IRS to assess any additional tax for tax year 2012 expired in 2016 unless the statute for your return was extended for some reason (such as an audit still in process). So, it is too late to amend that return. For tax year 2020, the regular statute of limitations for assessment expires on April 15, 2024, so you could still amend that return to report interest on bonds that matured in that year.
It is possible that Treasury didn't issue you 1099's for the redemption of these bonds in 2023 because they had matured in prior tax years.
As @VolvoGirl noted, interest on matured bonds is taxable in the year they mature, if not reported annually, or at the time redeemed if before they matured. From IRS Publication 550":
Reporting options for cash method taxpayers.
If you use the cash method of reporting income, you can report the interest on Series EE, Series E, and Series I bonds in either of the following ways.
1. Method 1. Postpone reporting the interest until the earlier of the year you cash or dispose of the bonds or the year in which they mature. (However, see Savings bonds traded, later.) Note. Series EE bonds issued in 1992 matured in 2022. If you have used method 1, you generally must report the interest on these bonds on your 2022 return. The last Series E bonds were issued in 1980 and matured in 2010. If you used method 1, you generally should have reported the interest on these bonds on your 2010 return.
2. Method 2. Choose to report the increase in redemption value as interest each year
See this TurboTax tips article for more information on investment bonds.
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