On 30 December 2018, the Italian Parliament approved Law No. 145 (the “2019 Budget Law”), published in the Official Gazette No. 302 of 31 December 2018. The law went into effect in 2019.
You can find the official, authoritative text of the law here
To save you time I have dowloaded the law in PDF form from that web site.
The relevant section begins on the page marked "46", but it's actually the 50th page in the PDF because they don't number some of the title pages.
In 2022 there were changes to the law that amends the 7% program. Comuni in designated seismic zones with a population not exceeding 20,000 are now eligible for the program. Finding the exact list of towns is tricky because the law does not enumerate them. Instead it refers you to the istat.it website to find the seismic zones. Then you have to find towns of under 20,000 population. I found a list of these towns and matched them to population data from istat.it.
See to get my list of those comuni.
Here is the 2022 law in PDF form from official government web site. The relevant part is on the page marked as 4 although it is actually the 8th page of the PDF because some title pages are unnumbered. The paragraph begins, "Art. 6-ter (Estensione del regime fiscale dei pensiona- ti neoresidenti)."
Italy and the United States signed an agreement to prevent double taxation of their respective citizens.
The Italian Finance Ministry has a web page about the 7% tax regime and discusses which comuni qualify. The Italian Tax Authority (Agenzia delle Entrate) has a way to submit questions and get official answers to tax questions. It is called the interpello.