The effects of volcanic eruptions - even big ones - don’t typically last more than a few years. So the Pinatubo blip is long since gone, and can’t be the explanation for the continued lower stratospheric temperatures.
A simple experiment: remove the 1990-2000 period from each of the graphs above (just cover it with your thumb or something). Does it still look like there’s a step change in the data?
The proper way to handle this would be a multiple regression on all the known factors affecting stratospheric temperatures, allowing one to “subtract out” the estimated effect of volcanoes to better reveal the underlying trend. That’s been done a few times for the troposphere/surface (unsurprisingly, once ENSO and volcanoes are accounted for, the so-called “pause” disappears entirely), but I’m not aware of anyone doing the same for the stratosphere.
