The pion-beam cancer therapy machines also confirm the equation. (And time-dilation, to boot.)
I used to argue with those in my class who played with the particle accelerator instead of doing their philosophy homework. That was going on half a century ago. How would they measure decay of a meson? Would it be by noting where it happened and thereby that it must have moved farther down the tube because it was going close to c and its timescale was stretched out of whack? Or was it going faster than c and for that reason moved farther down the tube before decay? How can you tell the difference?