In the Torah, a father’s sister is called “your father’s sister” and a mother’s sister is called “your mother’s sister.” The term “aunt” is reserved for an uncle’s wife.
The prohibition here is similar to Mitzvah #191, the prohibition against sexual relations with one’s father’s wife, even if she is not his mother. This law applies to the uncle’s wife even after his death or their divorce.
The underlying rationale is similar to what we have said before: the Torah imposed extra safeguards regarding relatives since they’re always around and we are overly familiar with them. Here, it would not have the aspect of potential damage to the gene pool, but it would be a dishonorable act towards one’s uncle.
The Torah only states this law regarding the wife of a paternal uncle. The Sages extended it to apply to an aunt on one’s mother’s side as well. (See Talmud Yevamos 21a-b.)
This mitzvah applies in all times and places. It is discussed in the Talmud tractate of Yevamos on pages 21a-b. It is codified in the Shulchan Aruch in Even Ha’Ezer 15. This prohibition is #342 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #125 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.