Not only have yields declined on nominal US bonds but TIPs yields have hit all time record lows. Five year TIPS now yields minus 0.84% (-0.84%) and the ten year TIPS yields zero. That means the real rate of return on a 10 year AA+ rate security, with no inflation risk, is zero. For taxable investors the real return is, of course, negative.
That said, the distortions continue to abound with the yield of AAA munis exceeding US Treasuries on all maturities up to 20 years. When I compute the AAA / AA+ after tax yield curve munis come out ahead in all time buckets.