Alums are required to 2-Step for two years after the date that they are no longer considered a student. This two year period is made possible by being assigned an affiliation called SIS-Extended by the Student Information System, and allows former students to continue to access Cal Student Central.
When you graduate, you get a grace period of 274 days that allows you to continue to access many of the campus resources you are used to using. After your grace period, some campus resources will no longer be available to you.
See https://calnetweb.berkeley.edu/calnet-me/how-claim-your-calnet-id/grace-periods for more information about grace periods and the SIS-Extended affiliation.