Not my area of expertise, and I know we have some lawyers on the sub of which I am not one, but I did some reading as a laymen into this question following the President Elect's tweet about stripping flag burners of their citizenship.

The basic answer is that barring a renunciation of one's citizenship, it is very difficult in US law to declare someone to be a non-citizen.

The contemporary circumstances under which it is theoretically possible to strip someone of their citizenship are currently defined under 8 U.S.C. §§ 1481–1489 which is mostly derived, with some more recent amendments which you can see on the notes section of that page, from the Immigration and Naturalization act of 1952.

It reads, and I've bolded what I would view as bits most relevant to your question:

A person who is a national of the United States whether by birth or naturalization, shall lose his nationality by voluntarily performing any of the following acts with the intention of relinquishing United States nationality—

(1) obtaining naturalization in a foreign state upon his own application or upon an application filed by a duly authorized agent, after having attained the age of eighteen years; or

(2) taking an oath or making an affirmation or other formal declaration of allegiance to a foreign state or a political subdivision thereof, after having attained the age of eighteen years; or

(3) entering, or serving in, the armed forces of a foreign state if (A) such armed forces are engaged in hostilities against the United States, or (B) such persons serve as a commissioned or non-commissioned officer; or

(4)(A) accepting, serving in, or performing the duties of any office, post, or employment under the government of a foreign state or a political subdivision thereof, after attaining the age of eighteen years if he has or acquires the nationality of such foreign state; or (B) accepting, serving in, or performing the duties of any office, post, or employment under the government of a foreign state or a political subdivision thereof, after attaining the age of eighteen years for which office, post, or employment an oath, affirmation, or declaration of allegiance is required; or

(5) making a formal renunciation of nationality before a diplomatic or consular officer of the United States in a foreign state, in such form as may be prescribed by the Secretary of State; or

(6) making in the United States a formal written renunciation of nationality in such form as may be prescribed by, and before such officer as may be designated by, the Attorney General, whenever the United States shall be in a state of war and the Attorney General shall approve such renunciation as not contrary to the interests of national defense; or

(7) committing any act of treason against, or attempting by force to overthrow, or bearing arms against, the United States, violating or conspiring to violate any of the provisions of section 2383 of title 18, or willfully performing any act in violation of section 2385 of title 18, or violating section 2384 of title 18 by engaging in a conspiracy to overthrow, put down, or to destroy by force the Government of the United States, or to levy war against them, if and when he is convicted thereof by a court martial or by a court of competent jurisdiction.

(b) Whenever the loss of United States nationality is put in issue in any action or proceeding commenced on or after September 26, 1961 under, or by virtue of, the provisions of this chapter or any other Act, the burden shall be upon the person or party claiming that such loss occurred, to establish such claim by a preponderance of the evidence. Any person who commits or performs, or who has committed or performed, any act of expatriation under the provisions of this chapter or any other Act shall be presumed to have done so voluntarily, but such presumption may be rebutted upon a showing, by a preponderance of the evidence, that the act or acts committed or performed were not done voluntarily.

However as a result of the 1967 Supreme Court case Afroyim v. Rusk arguably the most important part of that law, which was added as an amendment in the 1980s to reflect the Supreme Court's ruling is that such actions must be undertaken with the intention of renouncing your citizenship and that the government cannot merely withdraw a person's citizenship as a result of them having taken any of those actions. As a result, since the 1967 court decision it has become "virtually impossible to lose American citizenship without formally and expressly renouncing it."

Prior to that, the ability of the federal government to strip someone of citizenship, to expatriate them, was poorly defined. At it's most specific, as described in that article, there was:

"an 1865 law [that] provided for the forfeiture of the “rights of citizenship” of draft-dodgers and deserters, but whether the statute meant to deprive such persons of citizenship or of their civil rights is unclear.1268 Beginning in 1940, however, Congress did enact laws designed to strip of their citizenship persons who committed treason,1269 deserted the armed forces in wartime,1270 left the country to evade the draft,1271 or attempted to overthrow the Government by force or violence.1272 In 1907, Congress provided that female citizens who married foreign citizens were to have their citizenship held “in abeyance” while they remained wedded but to be entitled to reclaim it when the marriage was dissolved.

None of those provisions seem likely to me to apply to American citizen heads of foreign states, particularly given how narrowly "treason" is defined under US law.

As a result, the only time period to my reading in which Congress had given itself such powers to strip citizens in the circumstances relevant to a foreign head of state would have been betweeen 1952 and 1967, and I am not aware of them ever having done so. Whether any of the figures you mentioned voluntarily renounced their citizenship is not something I'm familiar with.