What is the stated final destiny for the magnetar's magnetic field after crossing the event horizon?

It is completely subsumed by the newly formed black hole and vanishes from external observation

The transition of a magnetar into a black hole involves a complete gravitational collapse past the point of no return. According to the classical description of a black hole, the resulting singularity is characterized only by three external properties: its mass, its electric charge, and its angular momentum. The immensely powerful magnetic field, which defined the magnetar's entire existence and caused starquakes, is carried inward past the event horizon along with all other stellar material. Once inside this boundary, the field structure is no longer accessible or observable from the external universe, resulting in an 'unadorned' black hole.