

I’d argue they didn’t, they just changed.
There are 2 groups worth noting. Government and private.
Government assassination is still a thing. Israel has used it aggressively over the last few decades. There are also signs that china has too. That’s just off the top of my head. It’s also worth noting that drone strikes etc can fill the same roll as an assassin.
Private has definitely changed. I suspect the high profile assassinations have stopped. Low level ones just had to get a lot better at not looking like assassinations. The ever classic boating accident being a good example.
The change is mostly from improvements in policing. You can no longer just move to another city to escape the law.
It’s also worth noting that a lot of society has changed. It used to be that a country pivoted on its leader. Now, it’s a lot more reliant on formal structures. Taking out a leader doesn’t have the same, devastating effect it used to. Iran being a good example.






https://en.wikipedia.org/wiki/Tachyonic_antitelephone
Basically, an “instant” communication device can create paradoxes.
E.g.
Earth sends mars a instant message. Mars the relay’s it via normal radio to a passing ship/probe moving at 0.99C. That probe then sends an “Instant” message to Earth.
The final message will (depending on alignments etc) arrive before the original was sent. If the message the voids the reason for the original message to be sent then a paradox happens.
You can make similar things happen with any form of FTL communication. C isn’t technically the speed of light, it is the speed of information. It just happens that light has no other speed limiting, and so moves at maximum speed.