In Shadowlands Sesason 3, the cut off for Keystone Master will require 2500 score. This is because of the addition of Tazavesh, the Veiled Market to the pool of mythic plus dungeons. The dungeon will be split into two different dungeons score wise, so you will see the addition of two dungeons in terms of available mythic plus dungeons.
NOTE: There is NO change to how score is calculated.