There was a strong upward trend in the late 1800s, with Heads of State being blown up or shot all over the place. But it petered out.

I would posit that contemporary systems of domination are less personalised and more institutional. And VIP protection TTPs have been developed, with its own classes of specialists.

The USA has lagged the rest of the West in prevention of assassination attempts on Heads of State, but most of those may have been inside jobs...