I'm aware this likely crosses the 20 year boundary in regards to post 9/11 changes in the interpretations of the law. I'm still posting it because I figure the change might be rooted in the 2001 AUMF which could be covered by the scope of this subreddit.
In Ghost Wars Steve Kolls claims that in the 90s the targeted killing of Osama bin Laden - possibly with a Predator drone predecessor - was considered but ultimately rejected as an option because of questions of legality, instead the CIA opted for attempting to capture him with the help of local fighters (which had a far smaller chance of success and ultimately led nowhere).
But at a later point the CIA obviously started doing such targeted killings, including with drones.
I wonder: How did that change came to be?
We can do a quick & brief rundown up to that time;
Two things to keep in mind;
(1) There is no federal law as such prohibiting killings on foreign soil,
(2) International laws [ for example, Article 2(4) of UN Charter ] are rather indirect on this point, and US has typically bypassed them in different manners to interpret its actions as valid, since determinations are dependent case by case to make it a lawful killing, since, almost by definition, assassination is unlawful killing. )
After the Church committe and its influenctial investigation & report, published in 1976, and considering that various bills failed to pass through the Congress, last in 1980, and in 1970s ( with Helms and Colby ) CIA underwent internal policy changes ( I am not familiar with this all that much, but plenty of literature can be found ) which moved away from state-sponsored killings & assassinatons, we see a start in Executive orders ( revision of 11905, and enactment of 12306 singnificantly limiting such actions ), but the the final was 12333 issued by Regan at the start of his presidency, which lasted till Clinton in this form, more or less.
During the 1990s, with the Gulf war, we see this in action against Iraqi high command, but this is legally and politically different, since it was in times of war and acting under the UN Security Council. -- Indeed, the first significant derogation as such from the 12333 was under Clinton, who removed certain provisions and amended it, significantly in 1998, which, among others, allowed deadly force in counterterror operations, so prior to this, such actions would probably receive severe backlash. I am not familiar with the work referenced in the description, or in what manner it is paraphrased, but to my knowledge local groups were trained to carry out an operation after the missile attack narrowly failed, and that this was also later unsuccessful due to the subsequent regime change which significantly deteriorated internal situation.
Of course, post-9/11, together with Congress approval ( Resolution ) and anti-terror exception, such operations posed little issues. Anything after this falls under the 20-year rule. I will drop some literature below though, since this was a very brief overview, but I might be able to answer any follow-ups within here.
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Daniel B. Pickard. (2001). Legalizing Assassination? Terrorism, the Central Intelligence Agency, and International Law. Georgia Journal of International and Comparative Law 30(1).
Boyd M. Johnson III. (1992). Executive Order 12,333: The Permissibility of an American Assassination of a Foreign Leader. Cornell International Law Journal 25(2).
Banka, A., & Quinn, A. (2018). Killing Norms Softly: US Targeted Killing, Quasi-secrecy and the Assassination Ban. Security Studies, 1–39.