We're talking about long time periods, but my sense is that the real return of bonds (blue line) has slowed, while the real returns of equities (red line) has not.
If nothing else, it seems like bonds increasingly require investors to take on long duration risk (and more interest rate sensitivity) in order to realize positive real returns, while short-duration bonds barely or do not even keep up with inflation anymore: https://totalrealreturns.com/s/VFISX,VFITX,VUSTX for a duration comparison on Treasuries.
If nothing else, it seems like bonds increasingly require investors to take on long duration risk (and more interest rate sensitivity) in order to realize positive real returns, while short-duration bonds barely or do not even keep up with inflation anymore: https://totalrealreturns.com/s/VFISX,VFITX,VUSTX for a duration comparison on Treasuries.