I've thought about this a bit, and don't think it is likely or even possible that the spear will shoot out due to the pressure. The spear / keg has notches. The spear needs to be lifted and rotated after the spiral retaining ring is removed. I have been removing the spiral ring about halfway and the CO2 leaks out, then I fully remove the spiral retaining ring.
With pressure in the keg, I don't think one would be able to rotate the stem in order to remove it. I have removed the ring with a little pressure remaining, the spear pops up a bit and locks in place and cannot be rotated and removed, you need to push the spear down and bleed out the CO2 so you can rotate the spear.
All this said, always safer to open without any pressure, but I think they are designed in such a way to prevent removing the spear if there is pressure in the keg.