You are thinking of I Bonds aka inflation bonds. Backed by the full credit of the US Govt. Next round of I bonds will have about a 9% yield, fully guaranteed so long as the US Treasury doesn’t run out of money.
Downfalls: only allowed to buy $10k per year ($20k if married), MUST hold for minimum of 1 year, if you sell before 5 years you lose 3 months of interest payments. Interest payments pay monthly and compound twice a year.
The 10 year treasury note currently yields a little over 3%, price down nearly 20% for the year after a 40 year bull run (36 of last 40 years were positive return). But on 1/1/22 they were under 1.5% yield, so the amount of time it has taken the yield to double (and consequently, the time it has taken the price of bonds to fall) is pretty spectacular.