What is the name of the mission phase after the planetary flybys?

The mission phase after the planetary flybys was the Voyager Interstellar Mission.

Voyager 2’s planetary tour ended with its Neptune encounter on August 25, 1989. After completing the Neptune science sequence, NASA formally began the Voyager Interstellar Mission on October 2, 1989. The spacecraft no longer had another planned planet to visit, so its focus shifted toward the outer heliosphere and the space between the stars.

During this extended mission, Voyager 2 measured charged particles, cosmic rays, magnetic fields, and plasma waves in increasingly distant regions of the heliosphere. In 2018, it crossed the heliopause, the boundary where the Sun’s solar-wind bubble gives way to interstellar space.

The name can be confused with “heliophysics mission,” which describes the scientific subject more generally, or “Interstellar Probe,” a proposed future mission concept. Voyager Interstellar Mission is the specific official phase name used for Voyager 2’s post-planetary operations.