The Russian docking module undocked from the ISS in 2021 was Pirs. Also known as Docking Compartment 1, or DC-1, it served the Russian segment of the station for nearly 20 years.
Pirs launched on September 14, 2001, and docked with the nadir, or Earth-facing, port of the Zvezda service module three days later. It provided a docking port for Soyuz crew spacecraft and Progress cargo ships. It also functioned as an airlock for Russian cosmonauts conducting spacewalks in Orlan suits.
On July 26, 2021, Pirs was undocked while attached to the Progress MS-16 cargo spacecraft. The combined vehicle was then directed into a destructive reentry over the South Pacific. Its removal cleared the port for Nauka, a much larger Russian laboratory module that joined the ISS shortly afterward.
A common mix-up is Poisk, another Russian docking compartment still associated with the station. Poisk was added in 2009 and was not the module removed in 2021. Pirs supported 52 spacewalks during its service life.