The Eye of Agamotto is one of Doctor Strange's defining artifacts. Up through "Avengers: Infinity War," the amulet housed the Time Stone, one of the six Infinity Stones. During Thanos's invasion, however, Strange gave up the stone, after which Thanos used it in the Snap and then destroyed it along with the other Infinity Stones.
In "Doctor Strange and the Multiverse of Madness," however, Strange is clearly wearing the Eye of Agamotto again. The simplest explanation is that he has simply crafted a new amulet, without the Time Stone, as more of a symbolic reminder than an actual power source. The end of the movie seems to contradict that, though. Strange develops his "third eye," which is directly tied to his Infinity Stone-containing Eye of Agamotto in the comics. Without the Infinity Stone, how could this Eye of Agamotto have the same power to help Strange develop this supernatural "extra" sense of sight?