We still do execute spies, as long as their actions were sufficiently serious.

https://www.law.cornell.edu/uscode/text/18/794

18 U.S. Code § 794 - Gathering or delivering defense information to aid foreign government

(a) Whoever, with intent or reason to believe that it is to be used to the injury of the United States or to the advantage of a foreign nation, communicates, delivers, or transmits, or attempts to communicate, deliver, or transmit, to any foreign government, or to any faction or party or military or naval force within a foreign country, whether recognized or unrecognized by the United States, or to any representative, officer, agent, employee, subject, or citizen thereof, either directly or indirectly, any document, writing, code book, signal book, sketch, photograph, photographic negative, blueprint, plan, map, model, note, instrument, appliance, or information relating to the national defense, shall be punished by death or by imprisonment for any term of years or for life, except that the sentence of death shall not be imposed unless the jury or, if there is no jury, the court, further finds that the offense resulted in the identification by a foreign power (as defined in section 101(a) of the Foreign Intelligence Surveillance Act of 1978) of an individual acting as an agent of the United States and consequently in the death of that individual, or directly concerned nuclear weaponry, military spacecraft or satellites, early warning systems, or other means of defense or retaliation against large-scale attack; war plans; communications intelligence or cryptographic information; or any other major weapons system or major element of defense strategy.

We also don’t have parole for federal crimes, so when the statue says you maybe get sentenced to life, it doesn’t mean “you get at most fifteen years until a parole hearing” the way it does in Germany. It means you’re going to jail for the rest of your life.

They were living in Slovenia, though, and the Council of Europe requires members to not have the death penalty, so they wouldn’t have been executed. The only European country that isn’t in the Council of Europe and has the death penalty is Belarus. Well, and Russia withdrew, and the Kremlin did extrajudicial killings anyway when Russia was in the Council of Europe.

I also don’t think that Slovenia considered this to be at the upper end of the scale. They apparently, assuming that this was them, got a year and seven months. I don’t know Slovenia’s criminal code, but I am confident that whatever espionage law they have permits for more-severe penalties than that.