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.