From VG247: “Now that the Garden of Salvation raid has been completed, the Vex Offensive quest is now available for Shadowkeep players.
When you log in, youll receive a message asking your Guardian to meet with Ikora in the Tower to discuss the pending Vex invasion.”
Source: N4G PC Destiny 2: Shadowkeep – How to complete the Eyes on the Moon quest and unlock the Vex Offensive