Updated on July 29, 2019

Microsoft release two special servicing phases updates that depends on latest current branch version released:

• Phases 1 – Security and Critical updates: you receive both security and critical update in the first 4 months after a new current branch update released.
• Phased 2 – Security Update only: after phases one, you will receive security update only for 14 months as showing in the below figure:

Starting from Current Branch version 1710, support end after 18 months life cycle (4 months Security and Critical updates + 14 months Security Update only) 

References:

Microsoft: Support for Configuration Manager current branch versions