It is not a Federal law. It’s Executive Order 11905 promulgated by President Ford - which you think they’d be interested in dumping, until you find out what followed that up and from who.
11905 was followed up by Jimmuh Cahtah’s Executive Order 12036, which is a much ballyhooed ‘ban on assassination’ long enshrined by the left.
https://en.wikipedia.org/wiki/Executive_Order_11905
https://en.wikipedia.org/wiki/Executive_Order_12036
It was further restated by Reagan in Executive Order 12333.
https://en.wikipedia.org/wiki/Executive_Order_12333
There is no actual Federal law banning assassination that I am aware of.